| 100th Training Division (Leader Development) | airborne infantry division, United States Army infantry division, training division | Fort Knox | 1918 |
| 20th Century Animation | television production company, animation studio, film production company | Century City | February 1994 |
| 20th Television Animation | business | Los Angeles | May 19, 1999 |
| 2K | video game publisher, video game developer | Novato | 2005 |
| 2K Play | video game publisher | Novato | September 10, 2007 |
| 2K Sports | video game publisher | Novato | January 25, 2005 |
| 3Com | business, enterprise | Marlborough | 1979 |
| 3D Realms | video game developer | Garland | 1994 |
| 4Kids TV | television production company, television channel | — | September 14, 2002 |
| 7-Eleven | brand, business, gas station chain, convenience store chain | Dallas, Irving | 1927 |
| 80th Training Command | functional command, United States Army infantry division, airborne division, training division | Richmond | 1917 |
| 87North Productions | film production company | Santa Monica | April 1, 2019 |
| 91st Training Division (Operations) | United States Army infantry division, training division, training brigade | — | 1917 |
| A Band Apart | film production company | Los Angeles | 1991 |
| A. T. Cross Company | business, public company | Lincoln | 1846 |
| ABC-CLIO | book publisher, publishing house | Santa Barbara | 1953 |
| Abercrombie & Fitch | business, clothing store chain, public company | New Albany | 1892 |
| Abilene Christian University | church college, private not-for-profit educational institution, seminary | Abilene | 1906 |
| AbioMed | business, public company | Danvers | 1981 |
| About.com | media company, website | — | April 21, 1997 |
| Access Systems Americas | business | Reston | — |
| Ace Hardware | organization, business | Oak Brook | 1924 |
| Activision | video game publisher, video game developer, subsidiary company | Santa Monica | October 1, 1979 |
| Actors Studio | drama school | New York City | October 5, 1947 |
| Adam & Eve | pornography company, sex shop, pornographic film studio | Hillsborough | March 1971 |
| Addison-Wesley | book publisher, publishing house | Boston | 1942 |
| AddThis | social bookmarking, business | Vienna | 2006 |
| AdMob | software company, business | Mountain View | 2006 |
| Adobe | software company, type foundry, business, data controller, enterprise, public company | San Jose | February 28, 1982 |
| Advance Auto Parts | car parts shop chain, business, public company | Roanoke | 1932 |
| Advent International | business, private equity firm | Boston | 1984 |
| Aeronca | aerospace manufacturer | Cincinnati, Middletown | 1928 |
| Aetna | insurance company, business, enterprise, public company | Hartford | 1853 |
| Affirm Films | business | Culver City | 2008 |
| AGBO | production company | Los Angeles | March 27, 2015 |
| Ageia | business | Santa Clara | 2002 |
| Agilent Technologies | business, enterprise, public company | Santa Clara | 1999 |
| Ahwahnee Hotel | lodge | — | 1927 |
| AIGA | regulatory college, nonprofit organization, private press, professional association | New York City | 1914 |
| AIM alliance | joint venture, alliance | — | October 2, 1991 |
| Air Force Institute of Technology | staff college, engineering education | — | 1919 |
| Air Sunshine | airline | Fort Lauderdale | 1982 |
| Airbnb | website, business, hotel booking website, enterprise, public company | San Francisco | 2008 |
| Airtable | spreadsheet, website, business | San Francisco | 2012 |
| AK Press | book publisher, small press | Chico | 1987 |
| Aksys Games | business | Torrance | July 14, 2006 |
| Alamo Rent a Car | business | Clayton | 1974 |
| Alaska Railroad Corporation | national railway, Class II railroad | Anchorage | 1914 |