In Los Angeles, Leon became one of the most sought after sessions musicians as a piano, keyboard, and guitar player. He was amongst an elite group of such musicians that became known as "The Wrecking Crew." Some others inlcuded Glen Campbell, Hal Blaine, Carol Kaye, Tommy Tedesco, Earl Palmer, Joe Osbourne, Joe Porcaro, Bill Pittman, Billy Strange, Larry Knechtel, Steve Douglas, Jim Horn, and Dr. John.

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Originally assembled as the house band for producer Phil Spector's records in the early 1960s, this collection of musicians would go on to perform on Spector's records throughout the decade, backing everyone from Frank Sinatra to the Beach Boys to Bob Dylan.

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Along with many of the other Wrecking Crew musicians, Leon performed in the house band for the "T.A.M.I Show" in 1964, a two day concert event that was captured on film and distributed to movie theaters across the US. The event was headlined by many of the top musicians of the day, including Chuck Berry, The Beach Boys, James Brown, Marvin Gaye, Smokey Robinson, The Supremes, and The Rolling Stones.

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Notable records that Leon can be heard performing on include:

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• "Surf City" - Jan & Dean

• "Mr. Tambourine Man" - The Byrds

• "Help Me, Rhonda" - The Beach Boys

• "When Somebody Loves You" - Frank Sinatra

• "A Taste Of Honey" - Herb Alpert & The Tijuana Brass

• "Da Doo Ron Ron" - The Crystals

• "Danke Shoen" - Wayne Newton

• "Say It Isn't So" - Aretha Franklin