Gilliam graduated from State University of New York at Purchase in 1990. Gilliam has had a number of recurring roles on television, including during the seventh season of The Cosby Show as Aaron Dexter, boyfriend of Erika Alexander's character, and as Alan Deaton on Teen Wolf. His most prominent role, however, has been on The Wire. In 2008, he attracted attention for criticizing the show being overlooked for Emmy nominations despite its critical success. He has also had a number of guest roles in other shows such as CSI: Miami, Nurse Jackie, Skins, Person of Interest, and The Good Wife. His film credits include Private Sugar Watkins in the 1997 action film Starship Troopers and Sergeant Steven Altameyer in the 1996 film Courage Under Fire. He played Clayton Hughes on Oz. Gilliam has also acted on stage, playing Prince Edward in a 1993 production of Richard III, and as the lead in the Commonwealth Shakespeare Company's 2010 production of Othello and the 2011 production of Othello at Houston Shakespeare Festival directed by his wife Leah C. Gardiner. He also played Anthony in Anthony and Cleopatra for Houston Shakespeare Festival also directed by his wife. He has stated that he prefers acting on the stage although he loves his work in film and television: "Acting for the stage is a lot more fun to do. There is instant communication with the audience. An actor can tell whether or not he or she is connecting with an audience or whether they understand what's going on." In May 2014, Gilliam was cast as Gabriel Stokes in the fifth season of AMC's The Walking Dead and continues to play Dr. Deaton in Teen Wolf for MTV.
In the Eyes of a Stranger
Assault at West Point: The Court-Martial of Johnson Whittaker