Mabry, Bubba: a private investigator in Albuquerque, New Mexico by Steve Brewer

Mabry, Caroline: a police detective in Spokane, Washington by Jess Walter

MacAlister, Marti: a widowed black police detective in Lincoln Prairie, Illinois by Eleanor Taylor Bland

MacAllister, Frank: a swashbuckling private investigator in Brazil by Ritchie Perry

Ian MacArthur: by Selwyn Jepson

MacBean, Elsie “Beanie”: a freelance writer and a member of the Was hoe Tribe and her basset hound, Cruiser, in South Tahoe, Nevada by Sue Owens Wright

Macbeth, Hamish: police constable in Scotland by M.C. Beaton

Stacey MacBeth: cop in Chicago, Illinois, by Dave Case

MacBride, Grace: founder of Monkeewrench, a game software company in Minneapolis, Minnesota by P.J. Tracy

MacDonald, Devon: a private investigator in St. Paul, Minnesota by Nancy Baker Jacobs

MacDonald, Paul: ex-reporter and mystery writer in San Francisco. California by Julie Smith

MacDonald, Robert: an inspector/superintendent  by E.C.R. Lorac 

MacDonell, Dr. Duncan A.:   a veterinarian in Houston, Texas by Robert E. Armstrong

Frankie MacFarlane: geologist based in Tucson, Arizona, by Susan Cummins Miller

MacGowan, Maggie: a documentary filmmaker in Los Angeles, California by Wendy Hornsby

Isabel Macintosh: faculty dean, and Giles Sullivan, a retired attorney, in Vermont, in the Crossword Puzzle mysteries by Herbert Resnicow

Mackay, Kathryn: prosecuting attorney in northern California by Christine McGuire

MacKinnon, Scotia:   former police officer turned private eye in Friday Harbor, San Juan Island, Washington by Sharon Duncan

Brett Macklin: aka Mr. Jury, aka the .357 Vigilante in Los Angeles, California, by Ian Ludlow (Lee Goldberg)

Macklin, Peter: hit man for the mob in Detroit, Michigan by Loren D. Estleman

Mackey, Herbert: a sheriff in Blanco County, Texas by Ben Rehder

Captain Duncan Maclain: blinded by gas in WWI, working as a detective, assisted by his wife Rena, and Spud Savage, in New York City, by Baynard Kendrick

MacLeish, Cooper: a taxi driver and Vietnam vet in Chicago, Illinois by Sam Reaves

MacLeod, Chanse: a gay private investigator in New Orleans, Louisiana by Greg Herren

Enzo Macleod: Scottish biologist based in France, by Peter May

Declan (Mac) MacManus: struggling 30-something part-time private investigator living above a bar, in Southampton, New York, by Daniel Judson

Dusky MacMorgan: ex-Navy SEAL, in Florida, by Randy Striker (Randy Wayne White)

Elisha Macomber: 70-something Chairman of the Board of Selectmen of Penberthy Township, and proprietor of a fish market, in fictional Penberthy Island, Massachusetts, by Kathleen Moore Knight

Camilla MacPhee: who runs an advocacy agency for victims of violent crime, in Ottawa, Ontario, Canada, by Mary Jane Maffini

MacPherson, Annie: an attorney in Seattle, Washington by Janet Smith

MacPherson, Elizabeth: forensic anthropologist in the Southern US by Sharyn McCrumb

Alonzo MacTavish: debonair rogue in England, by Peter Cheyney

Madden, David: a postal inspector by Doris Miles Disney

Madden, John: Scotland Yard Inspector and shell-shocked veteran of World War I by Rennie Airth

Maddox, Ivor: sergeant at the Wilcox Avenue Station in Hollywood, California by Elizabeth Linington

Madriani, Paul: defense attorney in California by Steve Martini

Madrid, Nick: a freelance journalist by Peter Guttridge

Magaracz, Nick: Private Eye in Trenton, New Jersey by Kate Gallison

Maglione, Gianna: lesbian police lieutenant in Washington D.C. by Penny Mickelbury

Maguire, Cormac: an Irish archaeologist in Ireland by Erin Hart

Maguire, Maggie: an ex-Pinkerton agent in 1870s San Francisco, California by Kate Bryan

Maigret, Inspector Jules: Maigret, Inspector Jules: a police inspector in Paris, France by Georges Simenon

Maigret, Madame: wife in Paris, France by Georges Simenon

Maitland, Anthony: English barrister drawn to murder cases by Sara Woods

Kyle Malachi: who publishes mystery book reviews under the pseudonym Stokes Moran, by Neil McGaughey

Malcolm, Richard: CIA analyst and grad student in Washington D.C. by James Grady

Steve “Snap” Malek: police reporter for the Tribune, in 1930s–1940s Chicago, Illinois, by Robert Goldsborough

Lord Ambrose Malfine: English aristocrat back from fighting the Greek War of Independence, by Jane Jakeman

Mallett, Don: thief and poacher in Medwell Fratorum, England by Frank Parish

Mallett, Inspector: in England by Cyril Hare

David Mallin and sometimes George Coe, private detectives in England, by Roger Omerod

Mallory: small-town student mystery writer in Iowa by Max Allan Collins

Blair Mallory: health club owner in North Carolina, by Linda Howard

Dev Mallory: Secret Service Agent in the post-Civil War US, by Ed Gorman

Mallory, Kathleen: NYPD cop in New York, New York by Carol O’Connell

Mallory, Stuart: a private investigator in Lansing, Michigan by Ronald Clair Roat

Malloy, Claire: small-town bookstore owner in Farberville, Arkansas by Joan Hess

Malloy, Hannah: one of an unlikely pair of 60-something sister, in Marine, California by Annie Griffin

Malloy, Laura: a journalist in Minneapolis, Minnesota by Mary Logue

Malloy, Vic: a private investigator in California by James Hadley Chase

Cotton Malone: ex-US Justice Department agent, by Steve Berry

Fritz Malone: private investigator, and bastard son of a former police commissioner, in New York City, by Richard Hawke (Tim Cockey)

Ivy Malone: little old lady who uses her inconspicuousness in her sleuthing, by Lorena McCourtney

Malone, John J.: a lawyer in Chicago, Illinois by Craig Rice

Malone, Scobie: family-man police inspector in Sydney, Australia by Jon Cleary

Malory, Sheila: a writer of literary criticism in Taviscombe, England by Hazel Holt

Man, George: 18th century parish watchman in London, England by Keith Heller

Manchu, Fu: a pursuer of world dominance, by Sax Rohmer

Mancini, Munch: 1970s ex-druggie and jailbird in Los Angeles, California by Barbara Seranella

Dante Mancuso: private investigator, AKA the Pelican, in the North Beach area of San Francisco, California, by Domenic Stansberry

Mangum, Cuddy: 1/2 of a pair of police detectives in a small town in North Carolina by Michael Malone

Manion, Terry: private eye in New Orleans, Louisiana by Dick Lochte

Mann, Tiger: a spy in New York, New York by Mickey Spillane

Manning, Mark: a gay journalist in Chicago, Illinois by Michael Craft

Mansfield, Patrick: a detective sergeant in Suffolk, England by Ann Quinton

Dr. Manson by Edwin Radford and M. A. Radford

Inspector Manson by Edwin Radford and M. A. Radford

Simon Manton: police inspector, later superintendent, in London, England, by Michael Underwood

Manwaring, Kevin: field producer in Los Angeles, California by Robert Irvine

Manziuk, Paul: Detective Inspector with homicide, in Toronto, Ontario, Canada by N. J. Lindquist

Mapstone, David: ex-cop, now unemployed college history teacher back in law enforcement, in Phoenix, Arizona by Jon Talton

Mappestone, Sir Geoffrey: a knight during the time of the Crusades by Simon Beaufort

Marcella, Aurelia: a young Roman innkeeper in Roman Britain in 91 AD by Jane Finnis

March, Colonel: head of Scotland Yard's fictitious Department D-3, the Department of Queer Complaints by John Dickson Carr

March, Jasmine: cookbook author and gourmand specializing in rich recipes in Georgetown, Washington D. C. by Nina Killham

John March: black sheep of a staid merchant-banking family, now an ex-deputy sheriff working as a private investigator in Manhattan, New York City, by Peter Spiegelman

March, Tobias and Lavinia Lake, lovers and partners-in-crime investigators, in Regency London, England by Amanda Quick

Josie Marcus: undercover store spy in St. Louis, Missouri, in the Mystery Shopper series by Elaine Viets

Lili Marino: moving from Brooklyn to Walden Corners, New York, in the Gourd Craft mysteries by Maggie Bruce

Peter Marsh: wheelchair-bound ex-policeman, and his daughter Georgia, who investigate unsolved murders in Kent, England, by Amy Myers

Markby, Inspector Alan: detective inspector in Cotswolds, England by Ann Granger

Markham-Johnson, Helen: actress recruited by the Secret Service in London, England by Eileen Dewhurst

Marklin, Peter: a toyshop owner and antique toy collector in Dorset, England by Neville Steed

Lieutenant Marks: detective in New York City, by Dorothy Salisbury Davis

Marley, Calista:  forensic pathologist and wife, in Cleveland, Ohio by Bill Pomidor

Marley, Plato: family physician and husband in Cleveland, Ohio by Bill Pomidor

Marlin, John: a game warden in Blanco County, Texas by Ben Rehder

Jake Maroc: top agent for the Quarry, a secretive US government agency, by Eric Van Lustbader

Marlowe, Philip: private eye on the "mean streets" of Los Angeles, California by Raymond Chandler

Marlowe, Philip: private eye on the "mean streets" of Los Angeles, California by Robert B. Parker

Marple, Miss: spinster living in St. Mary's Mead, England by Agatha Christie

Stevie Marriner: soap opera star, and former spouse Neil Gulliver, former newspaper crime reporter, in Los Angeles, California, by Robert S. Levinson

Marryat, Stephen: an attorney in England by Margaret Leek

Maggie Mars: struggling screenwriter and former investigative journalist, in Los Angeles, California, by Cady Kalian

Marsala, Cat: freelance investigative journalist in Chicago, Illinois by Barbara D'Amato

Marsden, Grace: a writer of children's books in a  lovely reclaimed marsh compound called Pine Marsh, Illinois by Luisa Buehler

Marsh, Harry: chief inspector in England by Jo Bannister

Marshall, Neil: graduate student in creative writing, struggling poet and part-time chef and caterer in Houston, Texas by Tim Hemlin

Martens, Harriet: a detective superintendent in Greater Birchester, England by H. R. F. Keating

Martin, Cezanne: a cop in Fort Worth, Texas by Laurie Moore

Martin, Christine: a 40-something cub reporter in England by Annette Roome

Martin, Dorothy: American schoolteacher retired in England by Jeanne M. Dams

Lizzie Martin: companion to a wealthy widow slum landlord, in 1860s London, England, by Ann Granger

Pepper Martin: tour guide at a cemetery, who can talk with a mob boss ghost, in Cleveland, Ohio, by Casey Daniels

Rina Martin: retired actress, who played a private investigator in a television series, in Frantham, England, by Jane Adams

Martin, Saz: a lesbian private investigator in London, England by Stella Duffy

Martindale, Thomas: a college journalism professor in Oregon by Ron Lovell

Harry Martineau: detective inspector, later Detective Chief Inspector, in fictional Granchester, “Metropolis of the North,” England, by Maurice Procter

Martinelli, Kate: homicide detective in San Francisco, California by Laurie R. King

Rebecka Martinsson: tax attorney in Stockholm, called back to her hometown Kiruna, north of the Arctic Circle, in Sweden, by Åsa Larsson

Marvelli, Frank: a member of the New Jersey parole violators search team by Anthony Bruno

Marx, Albie: an ex-60s radical and a columnist for a radical magazine in the 90's in Los Angels, California by David Debin

Marx, Groucho: a movie star in Hollywood, California by Ron Goulart

Mary Helen, Sister: retired nun in San Francisco, California by Sister Carol Anne O'Marie

Mason, Perry: defense attorney in Los Angeles, California by Erle Stanley Gardner

Mason, Lou: an attorney in Kansas City, Missouri by Joel Goldman

Mason, Tom: a gay teacher in Chicago, Illinois by Mark Richard Zubro

George Masters: Scotland Yard DCI and later Chief Superintendent, and Bill Green, a Detective Inspector and later DCI, in London, England, by Douglas Clark

Masters, Molly: cartoonist and greeting card entrepreneur in Albany, New York by Leslie O’Kane

Masterson, Marla: a young British Professor of Anglo Saxon Literature in the United States by Peter Dunant

Masuto, Masao: private investigator in Beverly Hills, California by E.V. Cunningham

Matelli, Angela: an ex-Marine turned private investigator in Boston, Massachusetts by Wendi Lee

Mathilde of Westminster: physician and former lady-in-waiting, in the court of Edward II, in 14th century, London, England, by Paul Doherty

Matthews, Charles: a sheriff in Crawford County, Texas by D.R. Meredith

Matthews, Daphne: police psychologist in Seattle, Washington by Ridley Pearson

Matthews, Nell: a newspaper reporter in Grantham, Oklahoma by Eve K. Sandstrom

Whiskey Mattimoe: recently widowed, 30-something real estate broker, in the small resort town of Magnet Springs, Michigan, by Nina Wright

H.J. Mavity and Ben Spanner, ex-spouses in Connecticut, by Ron Goulart

Alex Mavros: half-Greek, half-Scottish private investigator in Greece by Paul Johnston

Max, Gaston: by Sax Rohmer

Maxim, Harry: Special Services Major, assigned to 10 Downing Street in London, England by Gavin Lyall

Maxwell, Lauren: an investigator for the Wild America Society in Anchorage, Alaska by Elizabeth Quinn

Maxwell, Sgt “Mama”: a detective in future Topeka, Kansas by Lee Killough

Michelle Maxwell and Sean King, Secret Service agents, in Washington, DC, and Virginia, by David Baldacci

Maxwell, Peter: a widowed teacher and golden-hearted cynic in England by M. J. Trow

John May and Arthur Bryant:, detectives in the Peculiar Crimes Unit, in London, England, by Christopher Fowler

Maybridge, Tom: a detective chief inspector in England by B. M. Gill

Mayo, Asey: former sailor & auto racer in Cape Cod, Massachusetts by Phoebe Atwood Taylor

Mayo, Gil: a police detective in Midlands, England by Marjorie Eccles

McAfee, Kerry and Terry:   twin fledgling investigators in Southern California by Jennifer Colt

Jo McAllister: proprietor of Jo’s Craft Corner, in the small town of Abbotsville, Maryland, in the Craft Corner mysteries by Mary Ellen Hughes

McAvoy, Ed: police chief in Ulster County, New York by Bill Stackhouse

McBride, Rex: a private eye in Los Angeles, California by Cleve F. Adams

McCabe, Cassidy: a psychotherapist and a calico cat in Oak Park, Illinois by Alex Matthews

McCain, Sam: an attorney in Black River Falls, Iowa by Ed Gorman

McCalastar, Sidney Adamson: an attorney in Littleton, Mississippi by Faye Snowden

McCaleb, Terry: retired FBI agent in Los Angeles, California by Michael Connelly

Duncan McCallum: Scottish prisoner transported to Colonial America, by Eliot Pattison

McCandler, Doan: a gay private investigator in San Francisco, California by Orland Outland

McCarron, Blue: independent social psychologist/PI near San Diego, California by Abigail Padgett

McCarthy, Gail: a veterinarian in the Santa Cruz Equine Practice, in Santa Cruz, California by Laura Crum

McCauley, Quint: ex-cop turned private investigator in a Chicago suburb in Illinois by D. C. Brod

McCleary, Mike: part of a wife & husband P.I. team by T. J. MacGregor

McCleet, Adam: ex-cop turned sculptor in Portland, Oregon by Rick Hanson

McClintock, Lara: an antiquities dealer and shop owner in Toronto, Ontario, Canada by Lyn Hamilton

McClintock, Shirley: a 50-something rancher in Colorado by B. J. Oliphant

Wilton McCleary: Civil War veteran and policeman, in 1870s Philadelphia, Pennsylvania, by Mark Graham

McCloud: a New Mexico marshal in New York by Collin Wilcox

McCone, Sharon: legal investigator/private eye in San Francisco, California by Marcia Muller

Andi McConnell: who turns 60, loses her job, and inherits a limousine, by Lorena McCourtney

McCool, Doug, a Vietnam vet and process server in San Francisco, California by Miles Archer

McCorkle, "Mac": a saloon owner in Bonn, Germany by Ross Thomas

McCoskey, Sunny: a chef in wine country, Napa Valley, California by Nadia Gordon

McCoskey, Sunny: a chef in wine country, Napa Valley, California by Nadia Gordon

McCracken, Blaine: an exiled agent by Jon Land

McCray, James “Mack”: a federal undercover cop in Stuart, Florida by Paul E. McElroy

McCrite, Carrie: a widow in the Ozark mountains of Arkansas in the Something to Die for Mysteries by Radine Trees Nehring

Dickson Mc’Cunn: grocer, and the Gorbals Die-Hards, in Scotland, by John Buchan

McCutcheon, Claude: a laid-back bachelor lawyer in the San Francisco-Bay Area, California by John A. Miller

Katie McDonald: mathematician and Sudoku whiz who returns to her hometown of Granville, New Hampshire, by Shelley Freydont

McDougal, James: homicide inspector, in Wingate, Connecticut by Hildegarde Dolson

McFaile, Korine: a landscaper and partner in Three Dirty Women Landscaping, Inc. in Pine Grove by Julie Wray Herman

McFarland, Mac: a burned-out reporter in Cape Cod, Massachusetts by Doug Kiker

McGarr, Peter: police officer in Dublin, Ireland by Bartholomew Gill

McGarvey, Kirk: superspy by David Hagberg

McGee, Travis: adventurer, philosopher and private eye in Fort Lauderdale, Florida by John D. MacDonald

McGrail, Nuala Anne: young Irish immigrant, psychic and singer in Chicago, Illinois by Andrew M. Greeley

McGrogan, Annie: an actor and private investigator in New York City by Gillian B. Farrell

McInnes, Tom: a young attorney in Coopers Bend, Alabama by Mike Stewart

McIntire, John: retired military intelligence officer and newly appointed township constable in St. Adele, Michigan by Kathleen Hills

McIntyre, Ross: small town high school biology teacher in Maine by Kate Clark Flora

Urbino McIntyre: American expatriate writer and amateur sleuth, and his friend Barbara, Countess da Capo-Zendrini, in Venice, Italy, by Edward Sklepowich

Austin McKenzie: veterinarian, and his wife Madeline, in Trumansburg in upstate New York, by Claudia Bishop

McKenzie, Greg: a retired Air Force criminal investigator in Nashville, Tennessee by Chester Campbell

McKall, Sam: a political advisor in Utah by Eric C. Evans

McKenna, Brian: police detective in New York, New York by Dan Mahoney

Owen McKenna: ex-San Francisco homicide inspector turned private investigator, and his Harlequin Great Dane, Spot, at Lake Tahoe, California, and Nevada, by Todd Borg

McKenna, Michael: detective chief inspector (DCI) in Wales by Alison G. Taylor

McKenna, Patience Campbell: 6-ft romance novelist turned crime writer in New York by Jane Haddam

McKenzie, Alex: a doctor in Maine by J. S. Borthwick

McKenzie, Ellie: a newly certified real estate agent returning to her old hometown of Santa Louisa by Kathleen Delaney

McKenzie, Joanna: former CID turned PI in Bath, England by Margaret Duffy

McKinney, Lee: a divorcee, returns to Michigan to work for her aunt's chocolate business by Joanna Carl

McKnight, Alex: a former cop turned rental agent, works on the Michigan's Upper Peninsula, on the shore of Lake Superior by Steve Hamilton

Mike McLean: freelance journalist in England, by Paul Adam

McLeish, John: DCI in London, England by Janet Neel

Michael McLoughlin: detective inspector, later retired, in Dublin, Ireland, by Julie Parsons

McMonigle, Judith: bed and breakfast owner in Seattle, Washington by Mary Daheim

McMorrow, Jack: a small-town editor in Androscoggin, Maine by Gerry Boyle

Christina McMullen: cocktail waitress turned professional psychologist, and homicide detective Jack Rivera, in Los Angeles, California, by Lois Greiman

McNally, Arch: playboy private eye in Palm Beach, Florida by Lawrence Sanders

McNally, Arch: playboy private investigator in Palm Beach, Florida by  Vincent Lardo.  Vincent Lardo continued the Arch McNally series after Lawrence Sanders’ death. 

Brian McNulty: bartender in 1980s Manhattan, New York City, by Con Lehane

Dennis McQueen: detective sergeant in New York City, by Robert J. Randisi

McQuinn, Rose: Inspector Faro's daughter, returns from the American Wild West to Edinburgh, Scotland by Alanna Knight

Logan McRae: detective sergeant in Aberdeen, Scotland, by Stuart MacBride

McTavish, Stoner: a lesbian travel agent in Boston, Massachusetts by Sarah Dreher

McWhinny, Tish: 70-something Vermont artist and painter by B. Comfort

Meade, Lois: cleans houses for pin money in a quaint village named Long Farnden, England by Ann Purser

Meadows, Jayne: celebrity crime-solver in Los Angeles, California by Steve Allen

Major Mearns and Sergeant Denny, special agents for William Pitt to watch George III, in England, by Gwendoline Butler

Medina, John: a CIA agent by Linda Howard

Patricia “Paddy” Meehan: crime reporter for the Scottish Daily News, in Glasgow, Scotland, by Denise Mina

Meiklejohn, Charlie: ex-arson investigator P.I. in New York by Kate Wilhelm

Susan Melville: spinster art teacher and painter, who becomes a freelance assassin, in New York City, by Evelyn E. Smith

Men at War Series: featuring the OSS during World War II by W.E.B. Griffin

Mendoza, Luis: lieutenant in the Los Angeles Police Department in California by Dell Shannon

Miguel Menendez: Indio investigator for the Bureau of Tourism, in San Luis Potosi, Mexico, by Suzanne Blanc

Menlo, Anne: a child psychologist in Phoenix, Arizona by Maxine O'Callaghan

Mercer, Philip: an engineer, geologist and eco-warrior by Jack Du Brul

Meren, Lord: chief investigator for Pharaoh Tutankhamen in Egypt by Lynda S. Robinson

Merlin, Jack: a surgeon and magician in Pittsburgh, Pennsylvania by James Tucker

The Great Merlini;  a stage magician by Clayton Rawson

Merrick, Michael: a newspaperman in 1840s Boston, Massachusetts by Mary Jo Adamson

Desmond Merrion: military intelligence officer who becomes a private detective, and Henry Arnold, a Scotland Yard inspector, in England, by Miles Burton (John Rhode)

Merrivale, Sir Henry: holder of one of the oldest baronetcies in England, physician, barrister, head of military intelligence for the war office in England by John Dickson Carr

Metellus, Decius Cecilius: would-be playboy son of an illustrious family, in the SPQR historical mysteries by John Maddox Roberts

Sofie Metropolis: Greek-American private investigator, in Queens, New York City, by Tori Carrington

Michaels, Laura: a retirement center social worker in Timberlake, Oklahoma by John Miles

Michaelson, Richard: is a retired diplomat with the Foreign Service Office in Washington D. C. by Michael Bowen

Middleton-Brown, David: solicitor in London, England by Kate Charles

Mikaelian, Garreth: vampire cop, first in San Francisco, California and later in Baumen, Kansas by Lee Killough

Milano, John: a private investigator by Stanley Ellin

Miller: bicycling enthusiast and reporter, transferred from Belfast to Crossmaheart, Northern Ireland by Colin Bateman

Christopher Miller: FBI agent in Jersey City, New Jersey, by Chris Grabenstein

Miller, Hannah (Granny Hanny), a mystery reader and quilt-making enthusiast, in Pennsylvania, in the Amish Country mysteries by Barbara Workinger

Miller, Lydia: 30-something fashion editor in New York, New York by Eleanor Hyde

Miller, Nick: Sgt. Central Division detective in London, England by Jack Higgins

Robin Miller: lesbian romance novel and travel writer turned private investigator, in New York City, by Jaye Maiman

Millholland, Kate: an attorney in Chicago, Illinois by Gini Hartzmark

Millhone, Kinsey: private eye in Santa Teresa, California by Sue Grafton

Mills, Todd: a gay TV news reporter in Minneapolis, Minnesota by R. D. Zimmerman

Milodragovitch, Milo: ex-Army spy and hard drinking private investigator in Montana by James Crumley

Milos, Lucio, Ramon, and Neil meet for lunch and take turns to tell their stories, in the Lunch series by Derek Hansen

Max Mingus: ex-cop private investigator, in Miami, Florida, and Haiti, by Nick Stone

Minogue, Matt: a sergeant in the Irish police detective in Dublin, Ireland by John Brady

Miracle, Tori: ex-New York City crime writer turned Pennsylvania novelist by Valerie Malmont

Mitchell, Meredith: Foreign Service officer in Cotswolds, England by Ann Granger

Mitchell, Benny: a brash detective constable in Cloughton, Yorkshire, England by Pauline Bell

Mitchell, Michelle “Mitch”: a deep-sea diver and bar owner in Huron Harbor, Michigan by Doug Allyn

Mitchell, Scott: private investigator in England by John Harvey

Mom, Dave's mother, in Mesa Grande, Colorado by James Yaffee

Monaghan, Tess: private investigator in Baltimore, Maryland by Laura Lippman

Monarch, Greg: a lawyer in La Graciosa, California by Barry Siegel

Claude Monet: French impressionist painter, in early 1900s London, England, and Venice, Italy, by Jane Jakeman

Séraphin Monge: orphan returning to his hometown after WWI, in Provence, France, by Pierre Magnan

Adrian Monk: obsessive-compulsive police detective, in San Francisco, California, in the Monk TV novelizations by Lee Goldberg

Monk, Dittany Henbit: a garden club member in Lobelia Falls, Ontario, Canada by Alisa Craig

Monk, Osbert: author and husband in Lobelia Falls, Ontario, Canada by Alisa Craig

Richard Monk: lawyer in England, by Michael Underwood

Monk, Inspector William: amnesiac Victorian police inspector by Anne Perry

Monks, Carroll: an emergency-room doctor in San Francisco, California by Neil McMahon

Monkeyman, Dave the: sidekick and ASL signer for the president of a large security firm in Miami, Florida by Hialeah Jackson

Monroe, Louis: a Don Quixote-style cop, in Atlantic City, New Jersey by Bill Kent

Montalbano, Salvo: a police inspector in Sicily by Andrea Camilleri

Montana, Lane: a finder of lost things, in Atlanta, Georgia by Kay Hooper

Montero, Britt: newspaper crime reporter in Miami, Florida by Edna Buchanan

Montez, Luis: an attorney and former Chicano activist in Denver, Colorado by Manuel Ramos

Montford, Suzie: a Woman Police Constable in London, England during the Blitz by John Gardner

Freddie Montgomery: by John Banville

Montgomery, Grant: a 40-something retired mentalist and entertainer in Pittsburgh, Pennsylvania by Robert Aiello

Pat Montella: whose sleuthing is aided by her ability see and hear ghosts, on an estate near Fredricksburg, Virginia, by Elena Santangelo

Reuben Montoya and Rick Bentz, police detectives in New Orleans, Louisiana, by Lisa Jackson

Montrose, Bridget: a mother of small children in Palo Alto, California by Lora Roberts

Montrose, Claire: an amateur sleuth in Oregon by April Henry

Montrose, Jean: physician in Scotland by C. F. Roe

Moodrow, Stanley: a cop turned private eye in New York, New York by Stephen Solomita

Moon, Charlie: Ute police officer in Grant Creek, Colorado by James Doss

Moon, Louise: a mortuary technician at Charity's (The Charitable Hospital of St. Roche Without the Walls) mortuary, in London, England by Anne Morgellyn

Moon, Phyllida: television private eye in London, England by Eileen Dewhurst

Stella Mooney: Detective Sergeant in London, England, by David Lawrence

Moore, John Schuyler: turn-of-the-century journalist in New York, New York by Caleb Carr

Moore, Rebecca: a reporter by Judith Skillings

Sherry Moore: blind psychic who can relive a murder victim’s last moments, based in Philadelphia, Pennsylvania, by George D. Shuman

Toussaint Moore: black postal worker turned private detective, by Ed Lacy

Morales, Mira: a bookstore owner and psychic, in Tango Key, Florida by T.J. MacGregor

Cordelia Morgan: lesbian hit artist turned sleuth, in northern New Mexico and southern Colorado, by Bett Reece Johnson

Lillian Morgan: retired math professor in a retirement community in Chapel Hill, North Carolina, by Alan Cook

Morell, Charlie: a Cuban-American private investigator turned lawyer in Los Angeles, California by Alex Abella

Morelli, Teddy: college history professor in Seattle Washington by Linda French

Morgan, Dexter: is a blood spatter technician for Miami Dade Police Department.  He is also a sociopathic serial killer by Jeff Lindsay

Morgan, Rain: newspaper reporter in London, England by Lesley Grant-Adamson

Morgan, Taylor: medi-flight pilot in Alaska by Megan Mallory Rust

Paul Morganstein: lawyer, and his partner Ted Stevens, a hard-luck attorney, in Tallahassee, Florida, by Terry Lewis

Moriarty, Professor: archenemy of Sherlock Holmes in London, England by John Gardner

Morrelli, Nick: a sheriff in Platte City, Nebraska by Alex Kava

Christine Morris: forensic profiler from Canada, in the Hebrides, Scotland, by Maureen Jennings

Morrissey, John: a chief inspector and family man in Malminster, England by Kay Mitchell

Morse, Chief Inspector: inspector  in Oxford, England by Colin Dexter

Moseley, Hoke: a cop in Miami, Florida by Charles Willeford

Moses, Zen: a private investigator and cancer survivor in Santa Monica, California by Elizabeth Cosin

Mosley, Inspector: an inspector in the Lancashire and Yorkshire border country in England, written as John Greenwood

Moss, Phil: a Scottish Crime Squad officer in Glasgow, Scotland by Bill Knox

Moss, Wiley: graphic artist in Washington D.C. by Neal Barrett Jr.

Mr. Moto, a Japanese secret agent John P. Marquand

Mott, Angus: a police sergeant in Thames Valley, England by Clare Curzon

Mostyn, Colonel: by Mark Hebden

Mulcahaney, Norah: homicide detective in New York, New York by Lillian O'Donnell

Mulcay, Kate Kincaid: veteran newspaperwoman in Atlanta, Georgia by Celestine Sibley

Muldoon, half of a pair turn-of-the-century policemen in New York, New York by William Marshall

Mulheisen, “Fang”,  police detective in Detroit, Michigan by Jon A. Jackson

Tim Mulligan and Elsie Mae Hunt, professional archeologists and amateur sleuths, by Aaron Marc Stein

Munro, Dougal: 1940s brigadier in Ireland by Jack Higgins

Kent Murdock: hard-drinking crime photographer, and frequent co-star, tough guy Jack Fenner, in Boston, Massachusetts, by George Harmon Coxe

Murdoch, William: private investigator in 1890s Toronto, Ontario, Canada by Maureen Jennings

Murphy, Jack: an ex-cop turned private investigator in Los Angeles, California by David J. Sherman

Jimmie Murphy: undercover police officer in London, England, by Colin Bateman

Murphy, Molly: a Irish immigrant in 19th-century New York, New York by Rhys Bowen

Kate Murray: journalist, in Canada, by Emma Cole

Musing, Hiawatha and Antigone: siblings, professors, and part-time sleuths, in the Ivory Tower mysteries by David D. Nolta