| M&T Bank | bank, business, financial institution, public company | Buffalo | 1856 |
| M-Audio | business, musical instrument manufacturing company | Cumberland | 1998 |
| Macalester College | private not-for-profit educational institution, liberal arts college in the United States | — | 1874 |
| Macromedia | business, enterprise | San Francisco | February 25, 1992 |
| Macy's, Inc. | holding company, business, public company | Cincinnati, Macy's Herald Square | 1929 |
| Magnolia Pictures | film production company, film distributor | Dallas | 2001 |
| Mailchimp | online service, business, software | Ponce City Market | 2001 |
| MakerBot Industries | business, enterprise | New York City | January 20, 2009 |
| Malin Space Science Systems | space company | San Diego | 1990 |
| Mandalay Bay | skyscraper, casino, hotel | — | 1999 |
| Marathon Petroleum | business, gas station chain, public company | Findlay | 2009 |
| Marine Biological Laboratory | centre for marine biology, nonprofit organization, educational institution, research institute, laboratory | Falmouth | 1888 |
| Mark Levinson | business | United States | 1972 |
| Marriott International | hotel chain, business, enterprise, public company | Bethesda | October 8, 1993 |
| Mars, Incorporated | food manufacturer, chocolaterie | United States, McLean | June 23, 1911 |
| Marsh & McLennan Companies | business, public company | New York City | 1905 |
| Marvel Comics | comics publishing company, publishing house | New York City | June 1961 |
| MarVista Entertainment | privately held company | Los Angeles | 2003 |
| Marxists Internet Archive | digital library, website | — | 1990 |
| Mary Kay | multi-level marketing company | Dallas | 1963 |
| Mason Ewing Corporation | film industry, film production company | Los Angeles | 2011 |
| Massachusetts Bay Transportation Authority | public transport network, transport company | Boston | 1964 |
| Massachusetts College of Art and Design | public educational institution of the United States, art academy, higher education institution | — | 1873 |
| Massachusetts Institute of Technology | university of technology, space grant institution, private not-for-profit educational institution, university, Institute of Technology, academic publisher | Cambridge | April 10, 1861 |
| MasterClass | e-learning, streaming media, business | San Francisco | 2015 |
| Max's Kansas City | restaurant, nightclub, music venue | — | 1965 |
| Maxwell Technologies | business | San Diego | 1965 |
| Maybelline | business | New York City | 1915 |
| Mayo Clinic | nonprofit organization, clinic, educational institution, research institute | Rochester | 1889 |
| McCann | advertising agency | United States | 1902 |
| McCormick & Company | business, enterprise, public company | Hunt Valley | 1669 |
| McDonald's | trademark, fast food restaurant chain, organization, business, public company | Chicago | May 15, 1940 |
| McDonnell Aircraft Corporation | aerospace manufacturer | St. Louis | July 6, 1939 |
| McDonnell Douglas | former entity, defense contractor, aerospace manufacturer | St. Louis | 1967 |
| McKim, Mead & White | architectural firm | — | — |
| Meijer | supermarket chain, gas station chain | Grand Rapids | 1934 |
| Memorex | brand, business | Cerritos | 1961 |
| Mercer University | private not-for-profit educational institution, university | Macon | 1833 |
| Merchant Ivory Productions | film production company | London | 1961 |
| Mercury Marine | business | Fond du Lac | 1939 |
| Meredith Corporation | media conglomerate, business, public company | Des Moines | 1902 |
| Merrill | business, enterprise, credit institution | New York City | January 6, 1914 |
| Mesaba Airlines | airline | Memphis | 1944 |
| Meta Platforms | online service provider, technology company, business, public company | Menlo Park | July 29, 2004 |
| Metra | transport company, transport network | Illinois | 1984 |
| Metro-North Railroad | commuter rail network, transport company | New York City | 1983 |
| MGM Resorts International | public company | Las Vegas | May 31, 2000 |
| MGM Television | television production company, business | Beverly Hills | 1956 |
| Miami University | public educational institution of the United States, public research university | Oxford | 1809 |
| Micro Instrumentation and Telemetry Systems | business | Albuquerque | 1970 |
| Micron Technology | integrated device manufacturer, business, public company | Boise | October 1978 |
| MicroProse | video game developer | Hunt Valley | 1982 |
| Microsoft Store | retail chain, subsidiary company | Redmond | October 22, 2009 |
| Middle-earth Enterprises | business | Berkeley | 2022 |
| Middlebury College | private not-for-profit educational institution, private university | — | 1800 |
| Midwest Airlines | airline | Oak Creek | 1984 |
| Minerva University | university | San Francisco | 2012 |
| Mississippi State University | research university, unincorporated community, public educational institution of the United States, open-access publisher, land-grant university, public university | — | 1878 |
| Missouri Pacific Railroad | railway company, business | St. Louis | 1917 |
| Missouri University of Science and Technology | public educational institution of the United States, public university | — | 1870 |
| Mistwalker | video game developer, business, enterprise | Tokyo, Japan, Honolulu | 2004 |
| MIT Sloan School of Management | business school, faculty, publishing house | Cambridge | 1914 |
| MITRE Corporation | nonprofit organization, nonprofit corporation, business | Bedford | 1958 |
| Modern Library | publishing house, imprint | New York City | 1917 |
| Moderna | biopharmaceutical company, business, public company | Cambridge | 2010 |
| Molex | business, public company | Lisle | 1938 |
| Mondo Media | business | San Francisco | 1988 |
| Monmouth University | private not-for-profit educational institution, private university | — | 1933 |
| Monogram Pictures | Poverty Row | New York City, Los Angeles | 1931 |
| Monolith Productions | publishing house, video game developer | Kirkland | October 25, 1994 |
| Monsanto | multinational corporation, public company | St. Louis, Creve Coeur | 1901 |
| Monster Beverage | business, enterprise, public company | Minnesota | 1935 |
| Montclair State University | research university, public educational institution of the United States, public university | Montclair | 1908 |
| Moody's Investors Service | business | 7 World Trade Center | 1909 |
| Moon Express | privately held company | Mountain View | 2010 |
| Mooney International Corporation | aerospace manufacturer | Kerrville | 1929 |
| MoPar | business, enterprise | Chrysler Headquarters and Technology Center | 1933 |
| Morgan Creek Entertainment | film studio | Santa Monica | 1988 |
| MOS Technology | business, enterprise | Norristown | 1969 |
| Motorola Solutions | organization, business, enterprise, public company | Chicago | January 4, 2011 |
| Mount Holyoke College | private not-for-profit educational institution, women's college, liberal arts college in the United States | South Hadley | 1837 |
| MrBeast Burger | fast food restaurant chain, ghost kitchen, company | — | December 19, 2020 |
| MSA Safety | business, public company | Pittsburgh, Cranberry Township | 1914 |
| MTA Regional Bus Operations | bus company, bus-based transport system | New York City | 2008 |
| MTV Films | film production company | Los Angeles | 1996 |
| Murray State University | public educational institution of the United States, university | — | 1922 |
| MyFreeCams | adult camming website | — | March 19, 2002 |
| Mythic Entertainment | video game developer | Fairfax | 1995 |
| México. | business | Rome, San Clemente | 1999 |