Valenzuela, one of 22 children born to parents who had immigrated to the United States from Mexico, rose to prominence at Southern California racetracks before branching out to become nationally famous.
He had a powerful yet smooth style, comparable to that of the legendary Eddie Arcaro, and was known as being especially capable aboard feisty 2-year-olds.
"I can remember him coming to the old Tanforan Racetrack up in San Bruno; it was the first time I ever seen him ride," Ray York, 76, who won the 1954 Kentucky Derby aboard Determine, told The Times on Wednesday. "I watched him from the quarter-pole to the wire, and I said, 'There's a little son of a gun that can ride,' and boy he did."
Valenzuela, whose horse racing career began in 1951, might have announced his presence when he won the Californian aboard Porterhouse in 1956 at Hollywood Park. Porterhouse defeated Swaps, who then was considered all but unbeatable.
In the weeks leading to the 1958 Kentucky Derby, the focus was primarily on Silky Sullivan, who had a reputation for coming from behind to win races.
Television coverage helped fuel the Derby hype, and when Valenzuela prevailed aboard Tim Tam, before 118,000 spectators at Churchill Downs, he was called to appear on "The Ed Sullivan Show."
Valenzuela and Tim Tam won the Preakness that same year but fell short of claiming horse racing's Triple Crown. They finished second in the Belmont Stakes after Tim Tam suffered a bone fracture close to the finish line.
Valenzuela's fame increased in the 1960s, when he became a regular aboard Kelso, who was previously ridden by Arcaro. Valenzuela won 22 times in 35 tries aboard the gelding. Nineteen were stakes triumphs, and Kelso was horse of the year three times under Valenzuela.
It was aboard Forward Pass, however, that Valenzuela won the 1968 Kentucky Derby and Preakness -- and again finished second in the Belmont Stakes. The Derby victory came after race officials disqualified the first-place finisher, Dancer's Image, for a positive drug test.
Valenzuela's career waned in the 1970s and he experienced trouble obtaining premier mounts. In 1974, though, he won the Santa Anita Derby aboard Destroyer, a long shot that paid $89.90 for a $2 bet.
Around the track, Valenzuela was known as being personable and generous. At home, he was a doting father who stressed the importance of receiving an education.
"That's because he didn't have an education," said his daughter Patricia, the oldest of five Valenzuela children.
Born in McNary, Texas, on Dec. 24, 1934, Valenzuela was reared on a farm in Mexico after his parents had moved back. He herded sheep and cattle. When he was 12, he traveled to El Paso and got a job exercising horses and cleaning stalls.
Patricia Valenzuela added: "We all went to school and we all graduated, and we all handed our diplomas to him, and that was one of his biggest thrills -- more than winning a $100,000 race."
Valenzuela retired from racing in 1980 and last year was elected to horse racing's hall of fame by the historic review committee.
Valenzuela, who had recently been battling diabetes, is survived by daughters Patricia and Diana and sons Milo Jr., John and Richard. His wife, Rosa Delia, died in 1999.
Funeral arrangements are pending.