Um Ki-joon made his acting debut in 1995 in the stage play Richard III, though he later became best known as a musical theater actor, starring in Singin’ in the Rain,The Sorrows of Young Werther, Grease, The Three Musketeers, Jack the Ripper,The Count of Monte Cristo, Catch Me If You Can, and Rebecca.

Um’s popularity increased after he began playing supporting roles in television dramas such as Life Special Investigation Team, Worlds Within, Hero, Dream High,Scent of a Woman, and Phantom. He was also the leading actor in Good Job, Good Job and The Virus.

Um played the villain in his first film Man of Vendetta, opposite Kim Myung-min. This was followed by a role as Lee Ki-cheol, a detective investigating serial murders in horror film Killer Toon, opposite Lee Si-young.