Kennedy died Sunday in Boise, Idaho, said his grandson, Cory Schenkel.
"He passed Sunday morning, due to old age and some health issues," Schenkel said.
The New York-born Kennedy came by his military gravitas honestly; he served in World War II and spent 16 years in the U.S. Army, many of them with Armed Forces Radio. In the 1950s, he was an adviser to Phil Silvers' "Sgt. Bilko" show and then started getting acting roles.
Among his early notable films were 1963's "Charade," in which he played one of the criminal gang threatening Audrey Hepburn's character, and 1965's "The Flight of the Phoenix," in which he played a passenger on James Stewart's stranded airplane.
He broke through with 1967's "Cool Hand Luke," in which he played Dragline, a convict who resists, and then becomes friendly with, Paul Newman's Luke character.
"The marvelous thing about that movie was that as my part progresses, I changed from a bad guy to a good guy," Kennedy said in 1978, according to the Hollywood Reporter
. "The moguls in Hollywood must have said, 'Hey, this fellow can do something besides be a bad guy.' "
His performance won him an Oscar for best supporting actor.
Kennedy was, by then, a go-to character actor. He played Joe Patroni, the mechanic and problem solver who was the one constant in the 1970s "Airport" movies. He also starred in 1974's "Earthquake," another disaster flick, about a quake that hits Los Angeles.
Meanwhile, he had a number of TV roles, including starring in the short-lived '70s series "Sarge" and "The Blue Knight."
But he also had a sense of humor about himself, which came in handy given his beefy, 6-foot-4 frame.
He and Raymond Burr were friendly -- Kennedy guest-starred on both "Perry Mason" and "Ironside" -- and Kennedy told the Smashing Interviews website
that they used to break each other up.
"Raymond and I were big men," he recalled, noting their height and that "both of us weighed more than a ton and a half each."
But they found their size entertaining, not intimidating.
"If you can picture this, we'd talk about being big guys, and we would giggle. Now, here was 600 pounds of guys sitting between shots in a couple of chairs and almost breaking them because we're giggling at jokes. Over all of Raymond's movies and TV shows, I remember him as a guy who'd giggle with me and didn't care."
Kennedy also got to laugh with Albert Brooks, playing himself in 1981's "Modern Romance," and Leslie Nielsen, co-starring as Detective Captain Ed Hocken in the "Naked Gun" movies.
"I love comedy perhaps more than anything else," he told Smashing Interviews.
He didn't stop acting until he was in his 80s, by then doing a seven-year stint on the soap "The Young and the Restless." The Internet Movie Database lists more than 180 acting credits.
"My grandpa loved acting. Every opportunity he had, he enjoyed with such a passion," his grandson said. "It was definitely amazing to see that passion. Acting was natural to him."
Late in life, Kennedy wrote a memoir, "Trust Me," in which he described growing up lonely in New York and the joy he found in acting.
"I considered the time I spent acting a gift from the beyond," he told interviewer Brad Berkwitt
. "It was what I could do best."