In the 1950s, in addition to acting in genre films of all types, he is known for having appeared in several classic science fiction films, including The Man from Planet X (1951), The Incredible Petrified World (1957), The Astounding She-Monster (1957),[2] From the Earth to the Moon (1958, narrator), Beyond the Time Barrier (1960), and The Hideous Sun Demon (1958),[3] which Clarke wrote, directed and produced.

He also appeared in the cult film Captain John Smith and Pocahontas (1953) as John Rolfe, Clarke revealed in his 1996 autobiography To 'B' or Not to 'B' (co-written by Tom Weaver) that he made The Hideous Sun Demon for less than $50,000, including $500 for the rubberized lizard suit he wore.

He made two guest appearances on Perry Mason, including the role of circus co-owner and murderer Jerry Franklin in the 1960 episode, "The Case of the Clumsy Clown" and as Jack Harper in the 1957 episode "The Case of the Crooked Candle".

Other television appearances included The Lone Ranger, The Cisco Kid, Men Into Space, The Man and the Challenge, Hawaiian Eye, 77 Sunset Strip, Wendy and Me, General Hospital, Marcus Welby, M.D., Dragnet, Adam-12, Sea Hunt, Ripcord, Sky King, Checkmate, M Squad, Daktari, Baa Baa Black Sheep, Hawaii Five-O, Tabitha, Trapper John, M.D., Fantasy Island, Dallas, Simon & Simon, Knight Rider, Murder She Wrote, Matt Houston, Hotel, Dynasty, Falcon Crest, and dozens of others.

Clarke's last appearance was in the movie The Naked Monster, a send-up of the classic science fiction films of the 1950s, in 2005.