His big break came when he was just 21 with a recurring role as Aaron Dexter on the hit sitcom The Cosby Show.
Since then, he's been a regular or recurring presence on several hits including the Law and Order franchise and HBO's prison series OZ.
Prior to his current role, Gilliam was best know to television audiences as Sgt. Ellis Carver on HBO's The Wire.
He garnered critical acclaim for his big screen portrayal of Sgt. Steven Altameyer, a guilt-ridden soldier, in Courage Under Fire in 1995.
In 2014, Gilliam will start work on AMC's The Walking Dead. He was the second Teen Wolf actor to do so. Melissa Ponzio had a recurring role on TWD in 2013. As Ponzio did, Gilliam appeared in both shows simultaneously.