Klein apparently suffered a heart attack at his Rancho Santa Fe home this morning. He was taken by ambulance to Scripps Memorial Hospital in La Jolla, but efforts to revive him failed and he was pronounced dead at 5:35 a.m., hospital spokeswoman Edie High said.
High said Klein's wife and a nephew were with him at the time of his death.
Klein had a history of heart problems, and an autopsy was planned.
"He made a great contribution to the league, not only in San Diego, but on the television committee, where he was a visionary," Commissioner Paul Tagliabue said of Klein at the NFL meetings in Orlando, Fla. "He was a valuable and valued owner and he will be missed."
"The world of sports will miss Gene greatly. He was highly successful in all his endeavors and well respected in the National Football League," said Alex Spanos, the Chargers owner who bought the club from Klein in August, 1984, for about $80 million.
Klein had owned the NFL club since Aug. 25, 1966, when he headed a group that purchased the team for $10 million from Barron Hilton.
AFter he sold the club to Spanos, Klein became heavily involved in thoroughbred racing. With D. Wayne Lukas training his horses, Klein became one of the sport's leading owners, and one of his fillies, Winning Colors, captured the 1988 Kentucky Derby.
On Nov. 6, 1989, Klein sold his 114 horses for $29.6 million at Keeneland's November Breeding Stock sale, saying he wanted to spend more time with his wife.
Besides owning a Kentucky Derby champion, Klein won 11 Eclipse awards and his horses won seven Breeders' Cup races.
"I've got an awful lot of trophies to look at now and a lot of great memories," Klein said when he sold his stable. He estimated then that his horses had won more than 300 races and $25 million in the 6 1/2 years he was involved in the sport.
Klein said Winning Colors' Kentucky Derby victory was his biggest thrill in sports.
"I never won a Super Bowl, so I can't relate to it," he said. "But one of 28 teams is going to win the Super Bowl every year, but only one of 50,000 foals can win the Kentucky Derby and the odds are pretty big.
"We were very fortunate in having a great racehorse, especially a filly, win."
Lukas was stunned by Klein's death.
"I just talked to him yesterday. We were going to get together and have lunch Tuesday and he said 'great.' " Lukas said at his stables at Santa Anita in Arcadia.
NFL club owners heard of Klein's death at their meeting.
Klein is survived by his wife, Joyce. He also has a son, Michael Klein, and a daughter, Randee.