Johnson died yesterday morning at his home in Washington State of natural causes, said his agent, Mike Eisenstadt.
Johnson was a busy but little-known character actor when he was cast in the slapstick 1960s comedy about seven people marooned on an uncharted Pacific island.
He played high school science teacher Roy Hinkley, known to his fellow castaways as The Professor. There was seemingly nothing he couldn't do when it came to building generators, short-wave radios and other contraptions from scraps of flotsam and jetsam he found on the island.
During its three-season run on CBS, critics repeatedly lambasted the show as insipid. But after its cancellation in 1967, it found generations of new fans in reruns and reunion movies.
Also Read
One of the most recent of the reunion films was 2001's "Surviving Gilligan's Island: The Incredibly True Story of the Longest Three-Hour Tour in History," in which other actors portrayed the original seven-member cast while Johnson and two other surviving cast members narrated and reminisced.
"Parents are happy to have their children watch it," he said. "No one gets hurt. No murders. No car crashes. Just good, plain, silly fun - that's the charm."
He admitted he had trouble finding work after "Gilligan's Island," having become typecast as the egg-headed professor.
But he harboured no resentment for the show, and in later years he and other cast members, including Bob Denver, who had played the bumbling first mate Gilligan, often appeared together at fan conventions.
Johnson, Dawn Wells and Tina Louise were the last of the cast's survivors. Wells played vacationing farm girl Mary Ann Summers and Louise was sexy movie star Ginger Grant.
Before "Gilligan's Island," Johnson had appeared in dozens of films and television shows. His TV credits included "77 Sunset Strip," ''Gunsmoke," ''Rawhide," ''Wagon Train," ''The Lone Ranger," ''The Twilight Zone," ''Ben Casey," ''Hawaiian Eye" and "Death Valley Days."
He also appeared in more than two dozen feature films, including "MacArthur," ''The Greatest Story Ever Told" and cult science fiction favourites such as "It Came From Outer Space.