Kiel broke into films in the early 1960s with Eegah (1962), which was later featured on Elvira's Movie Macabre and Mystery Science Theater 3000, as were The Phantom Planet and The Human Duplicators. He also produced, co-wrote, and starred in The Giant of Thunder Mountain. Kiel appeared as the towering — and lethal — assistant Voltaire to Dr. Miguelito Loveless in first-season episodes of The Wild, Wild West. He later appeared in the episode "The Night of the Simian Terror" as Dimas, the outcast son of a wealthy family, banished because of birth defects that distorted his body and apparently affected his mind. This episode is significant because it allowed Kiel the opportunity to really act rather than just look intimidating. Kiel also had a cameo role in a 1961 episode of The Rifleman.