He is best known for his portrayals of Ellis Carver on The Wire, Clayton Hughes on Oz, Dr. Alan Deaton on Teen Wolf, and Father Gabriel Stokes on The Walking Dead.

[3][4] 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.

[6] He has had a number of guest roles in other shows such as CSI: Miami, Nurse Jackie, Skins, Person of Interest, and The Good Wife.

In May 2014, Gilliam was cast as Father Gabriel Stokes in the fifth season of AMC's The Walking Dead.

[9] Gilliam has acted on stage, playing Prince Edward in a 1993 production of Richard III.