Sadler got his first win in 1979 at Golden Gate Fields racetrack in the San Francisco Bay Area. Since then he has developed into a prominent figure in California racing, and has won numerous important races.
At Hollywood Park on June 28, 2008, Sadler won three stakes races when Whatsthescript won the Grade II American Handicap, Dearest Trickski triumphed in the Grade II A Gleam Handicap, and Emmy Darling won the Landaluce Stakes. On December 26, 2010, he repeated this performance at Santa Anita, as Twirling Candy won the Grade I Malibu Stakes, Switch won the Grade I La Brea Stakes and Sidney's Candy won the Grade II Sir Beaufort Stakes. Sadler won 3 stakes races for the third time at Hollywood Park on July 17, 2012 when Scherer Magic won the Grade III Hollywood Juvenile Championship, Switch in the Grade II A Gleam Handicap, and Lady of Shamrock in the Grade I Hollywood Oaks.
Sadler in 2011 surpassed $70 million earned in purses. His purse earnings passed $100 million in 2015.
In November 2012, John Sadler won his 2,000th race when Rooster City won a claiming race at Betfair Hollywood Park.
Sadler has won training titles at Hollywood Park, Santa Anita, Del Mar.
Other top runners for Sadler include Alpha Kitten, Appealing Missy, Belmont Cat, Black Mamba, Cost of Freedom, Dawn After Dawn, Dearest Trickski, Evita Argentina, Frost Free, Geronimo, Get Funky, Hasty Kris, Healthy Addiction, Lady of Shamrock, Musique Toujours, Noble Court, Oil Man, Olympic Prospect, Tasha's Miracle, Taste of Paradise, Three Peat, Tizbud, Track Gal, Victory Encounter and Zappa.