Mark started acting in film in 1977 with his role in the movie Julie. In 1979, Mark started acting on television with a two episode role on the television show, Barnaby Jones. In 1981, Mark has a 4 episode role in the television series, Hill Street Blues.
In 1997, Mark got casted for a show called Buffy the Vampire Slayer. He got the part of "The Master." His character one the should would become a fan favorite and he would continue to guest star until Buffy's last season. Mark also was in one episode of Buffy's spin-off show, Angel, in which he played the same character he did one Buffy.