Among his prominent roles is that of Robert, Will's ballet dancing boyfriend, on the NBC television series Will & Grace.
Sullivan frequently appears as Officer Andy Akers on the CBS television series CSI: Crime Scene Investigation.
His latest work includes a guest appearance on Modern Family as Mitchell's ex-boyfriend in the episode "My Hero" and a recurring role in the first season of the HBO series Big Little Lies.
Sullivan was a member of the First National Tour of the Broadway Musical Miss Saigon, and starred in the West Coast Premiere of Hello Again by Michael John LaChiusa at Noah Wyle's The Blank Theatre Company in Hollywood, California.
Sullivan's husband is actor David Monahan; they have one adopted child together.