Spinks, who had been suffering from prostate and other cancers for five years, died with his wife, Brenda Glur Spinks, by his side, the company said.
The St. Louis native, a former Marine who won a gold medal as a light heavyweight in the 1976 Olympics, was just 13 weeks and eight conflicts into his pro boxing career when he stepped into a ring from Ali on Feb. 15, 1978, in Las Vegas.
But Spinks -- a significant underdog who had never gone beyond 10 rounds -- went right after the champ, who was 36 years old and obviously about the downside of his career.
Spinks, then 24, had sufficient endurance to survive a 15th-round flurry out of Ali -- and provide one of his own at the closing seconds. The split decision moved to Spinks, who flashed his most renowned gap-toothed grin after getting the only fighter to have a name from Ali in the ring.
"In that struggle, everything clicked," promoter Bob Arum said Saturday. It wasn't that Ali wasn't at his best, but Leon shocked everyone with how great Leon was."
Suddenly, Spinks was the heavyweight champion of the world, memorably proclaiming afterward,"I'm not The Greatest. Only the newest."
But that would prove to be the highlight of Spinks' career. First, he had been stripped of the WBC title for needing to defend against leading contender Ken Norton. Instead, Spinks picked a rematch with Ali, who took back the WBA title in a unanimous decision in the Superdome in New Orleans seven weeks after their initial bout.
It was the last win of Ali's career.
He then dropped down to the cruiserweight division but dropped his sole title shot to Dwight Muhammad Qawi to some sixth-round TKO at March 1986. At points throughout his professional career, he'd Mr. T, the future film and television celebrity, as his bodyguard.
Spinks retired at age 42 after losing a unanimous decision to Fred Houpe in December 1995 and completed his career with an unremarkable record of 26-17-3 (14 KOs).
Spinks, who battled in wrestling and mixed martial arts later in his career, dealt with medical and financial issues later in life.
"I had been a bad young man," Spinks told the New York Daily News in 1997. "I never had nothing. All of a sudden I had something. I tried to do too much. I was mad. I used ton't care about nothing whatsoever. You think it's never going to end."
After moving to Las Vegas, Spinks married Brenda at 2011. Both were frequently seen at boxing-related activities, including Spinks' 2017 induction to the Nevada Boxing Hall of Fame.
"He was happy-go-luckythe salt of the earth," Arum said, chuckling at the memories. "Leon was nutty, but you couldn't get angry at the man. You couldn't help but love him although you shook your head at the way he acted."
In 2014, Spinks suffered intestinal damage and was hospitalized after swallowing a piece of bone. It led to multiple surgeries. In 2019he was in and outside of the hospital while undergoing treatment for prostate cancer.