In the mid-1960s, Leon took on a job as an arranger, creative developer, and assistant to producer and Viva Records owner, Snuff Garrett. In this role, Leon developed his skills as a songwriter, penning hits for Gary Lewis & The Playboys like "Everybody Loves A Clown," "She's Just My Style," and "This Diamond Ring."

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During this time, Leon also acted as arranger and conducter of the Midnight String Quartet's debut album, Rhapsodies For Young Lovers, which consisted of instrumental pop standards performed by a chamber quartet and rhythm section. The record spent 17 weeks on the Billboard charts, peaking at #17 in November 1966. This was originally meant to be a Leon Russell solo record, featuring himself on keyboards accompanied by the orchestra, but Garrett decided to brand it as The Midnight String Quartet instead.