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.

Then, in the low-budget, big-money-making 1980 disaster-movie parody “Airplane!” he was cast as a clueless doctor on board a possibly doomed jetliner. Some of his lines became catch phrases. (“Surely you can’t be serious,” one character says. Mr. Nielsen’s doctor replies: “I am serious. And don’t call me Shirley.”)

Critics and audiences alike praised his deadpan comic delivery, and his career was reborn.

“Airplane!” was followed by a television series, “Police Squad!” (1982), from the film’s director-writers, David Zucker, Jim Abrahams and Jerry Zucker.

It lasted only six episodes, but Mr. Nielsen, his goofy character, Lt. Frank Drebin, and the creators went on to three successful feature-film spinoffs.

The first, “The Naked Gun: From the Files of Police Squad!” (1988), was followed by “The Naked Gun 2 ½: The Smell of Fear” (1991) and “The Naked Gun 33 1/3: The Final Insult” (1994). Mr. Nielsen’s supporting cast in all three films included Priscilla Presley and O. J. Simpson.

Other filmmakers cast Mr. Nielsen in a variety of comedies, including “Repossessed” (1990), an “Exorcist” spoof withLinda Blair; Mel Brooks’s “Dracula: Dead and Loving It” (1995); “Spy Hard” (1996); and “2001: A Space Travesty” (2000).

None were received as well as the “Naked Gun” films, but Mr. Nielsen found a new continuing role as the paranoid, out-of-control president of the United States in “Scary Movie 3” (2003) and “Scary Movie 4” (2006).

In keeping with his adopted comic persona, when Mr. Nielsen in 1993 published an autobiography, “Naked Truth,” it was one that cheerfully, blatantly fabricated events in his life.

They included two Academy Awards, an affair with Elizabeth Taylor and a stay at a rehabilitation center, battling dopey-joke addiction.

In real life he was nominated twice for Emmy Awards, in 1982 as outstanding lead actor in a comedy series for “Police Squad!” and in 1988 as outstanding guest actor in a comedy series for an episode of “Day by Day,” an NBC sitcom about yuppies and day care.

Off screen, he was made an Officer of the Order of Canada, the country’s highest civilian honor, in 2002.

Leslie William Nielsen was born on Feb. 11, 1926, in Regina, Saskatchewan. The son of a member of the Royal Canadian Mounted Police of Danish heritage and a Welsh mother, he grew up in the Northwest Territories and in Edmonton, Alberta, where he graduated from high school. Jean Hersholt, the Danish-born actor and humanitarian, was an uncle.

Mr. Nielsen enlisted in the Royal Canadian Air Force before his 18th birthday and trained as an aerial gunner during World War II, but he was never sent overseas.

He began his career in radio in Calgary, Alberta, then studied at the Academy of Studio Arts in Toronto and at the Neighborhood Playhouse in New York. This led him to his television debut, in a 1950 episode of “Actors Studio,” an anthology series on CBS.

By the time Mr. Nielsen made his film debut, in 1956, he had made scores of appearances in series and performed in one Broadway play, “Seagulls Over Sorrento” in 1952, as a tyrannical Navy petty officer.

He continued to make guest appearances in television series throughout his career, and with great regularity through the 1970s.

And he did stage work, touring North America and Britain in a one-man show about the crusading lawyer Clarence Darrow.

His final projects included two 2009 comedies, “Stan Helsing” and “Spanish Movie.”

Mr. Nielsen married four times. His first wife (1950-56) was Monica Boyer; his second (1958-73) was Alisande Ullman, with whom he had two daughters; his third (1981-83) was Brooks Oliver. Those marriages ended in divorce.

In 2001 he married Barbaree Earl; a resident of Fort Lauderdale; she survives him, as do his daughters, Maura Nielsen Kaplan and Thea Nielsen Disney.

His elder brother, Erik Nielsen, who was deputy prime minister of Canada from 1984 to 1986, died in 2008.

In a 1988 interview with The New York Times, Leslie Nielsen discussed his career-rejuvenating transition to comedy, a development that he had recently described as “too good to be true.”

“It’s been dawning on me slowly that for the past 35 years I have been cast against type,” he said, “and I’m finally getting to do what I really wanted to do.”

Leslie W Nielsen