Phillips is a member of Alpha Phi Alpha fraternity. He attended the University of the Pacific as a communications major, but later transferred to the acting conservatory at New York University where he graduated with a BFA in acting in 1983.

From 1989 to 1991, Phillips portrayed U.S. Navy Lieutenant Martin Kendall on the popular NBC sitcom, The Cosby Show. He portrayed attorney Justus Ward on the ABC soap opera General Hospital from 1994 to 1998. Phillips was also a guest political commentator on News & Notes on most NPR radio stations between 2004 and 2009.

Phillips is an ambassador for the Sickle Cell Disease Association of America. He has spoken extensively about the disease and its effect on the family. He is also a member of the Screen Actors Guild, American Federation of Television and Radio Artists, Actors Equity Association, the Academy of Television Arts and Sciences, was National Co-Chair of the African American Steering committee for Bush Cheney '04, was named a member of the Republican National Committee's African American Advisory Board, was appointed by Governor Arnold Schwarzenegger to the state board of Directors of the California African American Museum and was named a 2005 Claremont Institute Lincoln Fellow.

Joseph Phillips' mother died when he was young. After this, he became close to the mother of Duane Evans, his cousin. Phillips is from Colorado. He and his wife Nicole have three sons, Connor, Ellis, and Samuel. In 2006, Phillips published an autobiography, He Talk Like A White Boy, which includes a foreword by Tavis Smiley. Phillips is a lover of cooking and Western films.