Leslie Nielsen, the Canadian-born actor who in middle age tossed aside three decades of credibility in dramatic and romantic roles to make a new, far more successful career as a comic actor in films like “Airplane!” and the “Naked Gun” series, died on Sunday in Fort Lauderdale, Fla. He was 84.

The cause was complications from pneumonia, said his agent, John S. Kelly.

Mr. Nielsen, a tall man with a matinee-idol profile, was often cast as an earnest hero at the beginning of his film career, in the 1950s.

His best-known roles included the stalwart spaceship captain in the science fiction classic “Forbidden Planet” (1956), the wealthy, available Southern aristocrat in “Tammy and the Bachelor” (1957) and an ocean liner captain faced with disaster in “The Poseidon Adventure” (1972).

In the 1960s and ‘70s, as his hair turned white and he became an even more distinguished figure, Mr. Nielsen played serious military men, government leaders and even a mob boss, appearing in crime dramas, westerns and the occasional horror movie.