| 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 |
| Narus | business, software | Sunnyvale | 1997 |
| NASA Space Science Data Coordinated Archive | data library | — | 1966 |
| National Academies of Sciences, Engineering, and Medicine | scientific organization, national academy, academy of sciences | Washington, D.C. | 1863 |
| National Academy of Engineering | academy of sciences | Washington, D.C. | 1964 |