| Lunar and Planetary Laboratory | graduate school, research institute | — | 1960 |
| Luxor Resort & Casino | skyscraper, hotel | — | October 13, 1993 |
| Lycoming | business | Williamsport | 1845 |
| 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 |
| Macromedia | business, enterprise | San Francisco | February 25, 1992 |
| Macy's | department store chain | New York City | October 28, 1858 |
| Macy's, Inc. | holding company, business, public company | Cincinnati, Macy's Herald Square | 1929 |
| 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 |
| Marathon Oil | business, enterprise, public company | Houston | 1887 |
| Marathon Petroleum | business, gas station chain, public company | Findlay | 2009 |
| Marine Corps University | naval force university, staff college, university | — | 1989 |
| Marlin Firearms | rifle factory, organization | North Haven | 1870 |
| Marmon Motor Car Company | business, automobile manufacturer | Indianapolis | 1902 |
| Marmot | business, enterprise | Santa Rosa | 1974 |
| Marquette University | private not-for-profit educational institution, university | Milwaukee | August 28, 1881 |
| Marquis Who's Who | publishing house | Berkeley Heights | 1899 |
| Masdar | business | Abu Dhabi | April 2006 |
| Massachusetts Bay Transportation Authority | public transport network, transport company | Boston | 1964 |
| Masten Space Systems | business | Mojave | 2004 |
| Match Group | business, company, public company | Kerrville | 2009 |
| Max's Kansas City | restaurant, nightclub, music venue | — | 1965 |
| Maximum Effort | film production company, company | New York City | 2018 |
| Maxis | video game developer | Emeryville | 1987 |
| Mayo Clinic | nonprofit organization, clinic, educational institution, research institute | Rochester | 1889 |
| McCann | advertising agency | United States | 1902 |
| McFarland & Company | book publisher, small press, publishing house | Jefferson | 1979 |
| McGraw-Hill Education | book publisher, business, enterprise | New York City | 1888 |
| McIntosh Laboratory | business, enterprise | Binghamton, New York City | 1949 |
| McKesson Corporation | business, enterprise, public company | Irving | 1994 |
| Meade Instruments | business, public company | Irvine | 1972 |
| Medium | dot-com company, service on Internet, blog software, social networking service, website, publishing platform, mobile app, online community | San Francisco | August 14, 2012 |
| Memorex | brand, business | Cerritos | 1961 |
| Men.com | pornographic film distributor, pornography website, pornographic film studio | — | 2003 |
| Mercer Automobile Company | business | New Britain | 1909 |
| Mercer University | private not-for-profit educational institution, university | Macon | 1833 |
| Mesa Airlines | regional airline | Phoenix | 1980 |
| Mesaba Airlines | airline | Memphis | 1944 |
| 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 |
| Metropolitan State University of Denver | public educational institution of the United States, public university | Denver | 1965 |
| MFS Investment Management | business | Boston | 1924 |