| M&T Bank | bank, business, financial institution, public company | Buffalo | 1856 |
| Macalester College | private not-for-profit educational institution, liberal arts college in the United States | — | 1874 |
| Mack Trucks | business, enterprise | Allentown, Greensboro | 1900 |
| Magnolia Pictures | film production company, film distributor | Dallas | 2001 |
| Magpul | business | — | — |
| MakerBot Industries | business, enterprise | New York City | January 20, 2009 |
| Mannes College The New School for Music | conservatory | — | 1916 |
| Mapbox | software company, startup company | Washington, D.C. | 2010 |
| Marine Corps University | naval force university, staff college, university | — | 1989 |
| Marmot | business, enterprise | Santa Rosa | 1974 |
| Marquis Who's Who | publishing house | Berkeley Heights | 1899 |
| Marriott International | hotel chain, business, enterprise, public company | Bethesda | October 8, 1993 |
| Martin Marietta | business, enterprise | Bethesda | 1961 |
| Marvel Studios | film production company | Burbank | 1993 |
| MarVista Entertainment | privately held company | Los Angeles | 2003 |
| Mary Kay | multi-level marketing company | Dallas | 1963 |
| Masdar | business | Abu Dhabi | April 2006 |
| Massachusetts College of Art and Design | public educational institution of the United States, art academy, higher education institution | — | 1873 |
| Masten Space Systems | business | Mojave | 2004 |
| Mastercard | organization, business, card association, enterprise, public company, payment system | Purchase | 1966 |
| Match Group | business, company, public company | Kerrville | 2009 |
| Matson, Inc. | business, public company | Honolulu | 1882 |
| Max's Kansas City | restaurant, nightclub, music venue | — | 1965 |
| Maxwell Technologies | business | San Diego | 1965 |
| Mayo Clinic | nonprofit organization, clinic, educational institution, research institute | Rochester | 1889 |
| Maytag | business, subsidiary company | — | 1893 |
| MCA Inc. | media company, entertainment company, conglomerate, company, public company | Chicago | November 10, 1958 |
| McDonnell Aircraft Corporation | aerospace manufacturer | St. Louis | July 6, 1939 |
| McDonnell Douglas | former entity, defense contractor, aerospace manufacturer | St. Louis | 1967 |
| McIntosh Laboratory | business, enterprise | Binghamton, New York City | 1949 |
| Mead Johnson | business, enterprise, public company | Glenview | 1905 |
| Meijer | supermarket chain, gas station chain | Grand Rapids | 1934 |
| Memorex | brand, business | Cerritos | 1961 |
| Mentholatum | business | Orchard Park | 1889 |
| Mentor Graphics | business, enterprise, public company | Wilsonville | 1981 |
| Merchant Ivory Productions | film production company | London | 1961 |
| Merck & Co. | pharmaceutical company, business, enterprise, public company | Rahway | 1891 |
| Meredith Corporation | media conglomerate, business, public company | Des Moines | 1902 |
| Meritor | public company | Troy | 1997 |
| Merrell | business | Rockford | 1981 |
| Merrill | business, enterprise, credit institution | New York City | January 6, 1914 |
| Mesa Airlines | regional airline | Phoenix | 1980 |
| Meta Platforms | online service provider, technology company, business, public company | Menlo Park | July 29, 2004 |
| MetLife | business, public company | New York City | March 24, 1868 |
| Metro Pictures | film production company, business | Los Angeles | 1915 |
| Metro-Goldwyn-Mayer | film production company, business | Los Angeles | April 17, 1924 |
| Metrolink | commuter rail network, railway company | Wilshire Grand Center | — |
| MFS Investment Management | business | Boston | 1924 |
| MGM Holdings | business | Beverly Hills | February 11, 2005 |
| Miami Dade College | college, public educational institution of the United States | — | 1959 |
| Miami Seaquarium | oceanarium, zoo | — | September 24, 1955 |
| Miami University | public educational institution of the United States, public research university | Oxford | 1809 |
| Michigan Technological University | public educational institution of the United States, university | Houghton | 1885 |
| 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 |
| Middle-earth Enterprises | business | Berkeley | 2022 |
| Midwest Airlines | airline | Oak Creek | 1984 |
| Millennium Media | film production company | Los Angeles | 1992 |
| Mindscape | video game publisher, video game developer | Novato | 1983 |
| Minneapolis Institute of Art | nonprofit organization, art museum, art academy | Minneapolis | 1883 |
| Minnesota State University, Mankato | public educational institution of the United States, public university | — | 1868 |
| Mises Institute | think tank, nonprofit organization, publishing house, educational institution | Auburn | 1982 |
| Mississippi State University | research university, unincorporated community, public educational institution of the United States, open-access publisher, land-grant university, public university | — | 1878 |
| Missouri University of Science and Technology | public educational institution of the United States, public university | — | 1870 |
| MIT Sloan School of Management | business school, faculty, publishing house | Cambridge | 1914 |
| Mobil | business, enterprise | Fairfax | 1966 |
| Mobile County Public School System | school district in the United States | — | 1836 |
| Modern Library | publishing house, imprint | New York City | 1917 |
| Moderna | biopharmaceutical company, business, public company | Cambridge | 2010 |
| Modulus Graphite | trademark, business, musical instrument manufacturing company, enterprise | Richmond | 1978 |
| Monmouth University | private not-for-profit educational institution, private university | — | 1933 |
| Monogram Pictures | Poverty Row | New York City, Los Angeles | 1931 |
| Monotype | privately held company, type foundry | Woburn | 1887 |
| Monsanto | multinational corporation, public company | St. Louis, Creve Coeur | 1901 |
| Monster Games | video game developer | Northfield | 1996 |
| Monster.com | public employment service, website, business, employment website | Guaynabo | 1999 |
| Montana State University | public educational institution of the United States, land-grant university, public university | Bozeman | 1893 |
| Montclair State University | research university, public educational institution of the United States, public university | Montclair | 1908 |
| Moody Bible Institute | private not-for-profit educational institution, higher education institution, Bible college, Christian organization | Chicago | 1886 |
| Moog Music | musical instrument maker, business | Asheville | 1953 |
| Mooney International Corporation | aerospace manufacturer | Kerrville | 1929 |
| Moravian University | private not-for-profit educational institution, liberal arts college in the United States | — | 1742 |
| Morgan Stanley | global systemically important bank, bank, business, enterprise, credit institution, public company | New York City | 1931 |
| MOS Technology | business, enterprise | Norristown | 1969 |
| Mosler Automotive | automobile manufacturer | Riviera Beach | 1985 |
| Mosrite | business | Arizona | 1956 |
| Motion Picture Patents Company | film production company | United States | 1908 |
| Motorola Solutions | organization, business, enterprise, public company | Chicago | January 4, 2011 |
| MSA Safety | business, public company | Pittsburgh, Cranberry Township | 1914 |
| MSCI | business, public company | New York City | 1969 |
| MTV Entertainment Studios | film production company | Hollywood | 1991 |
| Museum of Fine Arts, Houston | production company, nonprofit organization, art museum, museum | Houston | 1900 |
| Museum of Modern Art | production company, nonprofit organization, art museum, museum building, museum, museum of modern art | New York City | 1929 |
| MyFreeCams | adult camming website | — | March 19, 2002 |