Thomas died Thursday of respiratory failure at Sherman Oaks Hospital, said his stepdaughter, Julie Alexander.
Thomas began acting on Broadway in the early 1930s and soon ventured west to Hollywood, where he appeared in films including "A Dog of Flanders," "Boys Town" and "The Major and the Minor," as well as four Nancy Drew movies.
In 1950, he beat out actors including Jack Lemmon to win the title role of Tom Corbett, a Space Academy cadet who was training to become a member of the elite Solar Guard, 400 years in the future.
"Frankie looked like the all-American boy," said Jan Merlin, who played the wisecracking cadet Roger Manning. "Everyone in the room knew immediately this was the guy we were going to get."
The show aired on CBS, ABC, NBC and the DuMont network and spawned popular phrases including "Blast your jets," "Don't fuse your tubes" and "Spaceman's luck."
Thomas quit acting when the series ended in 1955 and, over the years, worked as a television and radio writer, bridge instructor and author of mystery novels, including "Sherlock Holmes and the Masquerade Murders."
He is survived by his stepdaughter and a stepson, James Aicholtz. At his request, Thomas was buried Tuesday in his "Tom Corbett, Space Cadet" costume.