Frankel died of cancer Monday at his home in Pacific Palisades, jockey agent Ron Anderson said. He was 68.
Frankel had been running his stable by phone for most of the year while undergoing treatment and concealing details of his illness from most of his colleagues, a remarkable feat in an industry fueled by gossip.
"He was a secretive guy," Hall of Fame trainer Bob Baffert said from Santa Anita. "He's from the old school of training -- nobody needs to know your business."
Frankel began his career at Belmont Park and Aqueduct in New York as one of the cheap hired hands who walk horses around the barn after morning workouts. He took out his trainer's license in 1966 and won his first race with Double Dash at Aqueduct that November.
He built an early reputation as "King of the Claimers," taking the cheapest horses and turning them into high-priced stakes winners.
Frankel saddled 3,654 winners and earned $227,949,775 during his 43-year career, according to Equibase. He was second only to D. Wayne Lukas in money won, and they were the only trainers to earn more than $200 million.
The Brooklyn-born Frankel oversaw a coast-to-coast string of horses, never losing his New York accent or brusque demeanor that came off as intimidating to most who sought him around the barn. He revealed a softer side only among his animals and close friends.
"Once you got him by himself, he was a lot of fun to be around," Baffert said.
Frankel enjoyed his greatest success this decade, winning four consecutive Eclipse Awards (2000-03) as the nation's leading trainer and five overall. His biggest client since the 1990s was Khalid Abdullah-owned Juddmonte Farms.
Frankel was a mentor to trainer Rick Dutrow Jr., who saddled Big Brown to victories in last year's Kentucky Derby and Preakness. The horse lost his Triple Crown bid in the Belmont, when he finished last.
"I used to love it when I would do something good, I would call Bobby first and say, 'Bobby, did you see that?' " Dutrow said Monday from New York. "When I didn't know what to do, he would be the first guy to call."