| American Society for Testing and Materials | standards organization, business, company, learned society | West Conshohocken | 1961 |
| American Society of Composers, Authors and Publishers | copyright collective, nonprofit organization, performing rights organization, business, company | United States, New York City | 1914 |
| American Society of Mechanical Engineers | engineering society, regulatory college, organization, academic publisher | New York City | 1880 |
| American Sociological Association | nonprofit organization, learned society, academic publisher | 1430 K Street | 1905 |
| American Tourister | brand, business | Rhode Island | 1933 |
| American Tower Corporation | business, enterprise, public company | Boston | 1995 |
| American Type Founders | type foundry, business | Elizabeth | 1892 |
| American University | research university, private not-for-profit educational institution, private university | — | 1893 |
| American Zoetrope | production company, film production company | San Francisco | 1969 |
| AmericanConnection | airline | Indianapolis | 2001 |
| Ameriprise Financial | business, enterprise, public company | Minneapolis | 1894 |
| Ameristar Jet Charter | airline | Dallas | 2000 |
| Ametek | public company | Berwyn | 1930 |
| Amkor Technology | service provider, business, enterprise, public company | Tempe | 1968 |
| Ampere Computing | fabless semiconductor manufacturer, business | Santa Clara | 2017 |
| AMR Corporation | business, enterprise | Fort Worth | October 1, 1982 |
| Amtrak | independent agency of the United States government, public transport network, railway company | Washington Union Station | May 1, 1971 |
| Ancestry.com Inc. | business, database | Lehi | 1983 |
| Angelo State University | public educational institution of the United States, university | San Angelo | 1928 |
| Anheuser-Busch | business, enterprise, public company | St. Louis | 1852 |
| Annapurna Interactive | video game publisher, company | Los Angeles | December 1, 2016 |
| Annapurna Pictures | film production company, entertainment company | Los Angeles | 2011 |
| Anthology Film Archives | cinematheque, movie theater, film archive | — | November 30, 1970 |
| Anti- | independent record label | Los Angeles | 1999 |
| Antioch College | Colleges That Change Lives, private not-for-profit educational institution, liberal arts college in the United States | — | 1850 |
| Apatow Productions | television production company, film production company | Los Angeles | 1999 |
| Apollo Global Management | business, private equity firm, enterprise, public company | Solow Building | 1990 |
| Apollo Group | business, enterprise, public company | Phoenix | 1973 |
| Apollo Theater | movie theater, nightclub, music venue | — | 1914 |
| Apple Inc. | corporation, technology company, business, enterprise, public company | Cupertino | April 1, 1976 |
| Apple Studios | production company, company | — | October 2019 |
| Arby's | fast food restaurant chain, business | Sandy Springs | 1964 |
| ARCO | oil company, business, gas station chain, enterprise | Los Angeles | 1966 |
| Arctic Cat | business, enterprise, public company | Plymouth | June 1960 |
| Arctic Coal Company | mining company | Boston | 1906 |
| Arista Networks | business, public company | Santa Clara | October 2004 |
| Arizona State University | public educational institution of the United States, university, open-access publisher | Tempe | 1885 |
| Armadillo Aerospace | privately held company | Mesquite | 2000 |
| ArmaLite | small arms manufacturing company | Geneseo | 1954 |
| Armand Hammer United World College of the American West | boarding school, academic institution | Montezuma | 1982 |
| Arris Group | business, enterprise, public company | Suwanee | 1995 |
| Arrow Air | airline | — | 1947 |
| Arrow Electronics | business, public company | Englewood | 1935 |
| Artforum | publishing house, magazine | New York City | June 1962 |
| Arthur J. Gallagher & Co. | business, public company | Itasca | 1927 |
| Ashland Inc. | business, enterprise, public company | Covington | 1924 |
| Ashton-Tate | business, enterprise | Torrance | August 1980 |
| ASM International | engineering society, publishing house, organization | ASM Headquarters and Geodesic Dome | 1913 |