In 1950, Riddle was hired by arranger
Les Baxter to write arrangements for a recording session with
Nat King Cole; this was one of Riddle's first associations with
Capitol Records. Although one of the songs Riddle had arranged, "
Mona Lisa", soon became the biggest selling single of Cole's career,
the work was credited entirely to Baxter. However, once Cole learned the true identity of the arrangement's creator, he sought out Riddle's work for other sessions, and thus began a fruitful partnership that furthered the careers of both men at Capitol.
During the same year, Riddle also struck up a conversation with
Vern Yocum, (born George Vernon Yocum) a big band jazz musician (brother of Pied Piper, Clark Yocum) who had transitioned into music preparation servicing Frank Sinatra. He also worked for Nat King Cole and other entertainers at Capitol Records. A
collaboration followed with Vern becoming Riddle's "right hand" as copyist and librarian for the next thirty years.
In 1952, Capitol Records executives viewed the up-and-coming Riddle as a prime choice to arrange for the newly-arrived
Frank Sinatra. Sinatra was reluctant however, preferring instead to remain with
Axel Stordahl, his long-time collaborator from his Columbia Records years. When success of the first few Capitol sides with Stordahl proved disappointing, Sinatra eventually relented and Riddle was called in to arrange his first session for Sinatra, held on April 30, 1953. The first product of the Riddle-Sinatra partnership, "I've Got The World On A String", became a runaway hit and is often credited with relaunching the singer's slumping career. His personal favorite, a Sinatra ballad album, "Only the Lonely."
He also supervised and arranged music for the legendary Judy Garland. Known for her distict style of singing, Riddle emphasized appraching a song "more gingerly." Showing the beloved singer that it was the surprises in a song that led to it's greatness, Garland recorded what is now considered her finest work. Never were her voice, the music arrangements or selections of songs better than the work she recorded fro Capitol alongside Riddle's direction.
Riddle was to stay at Capitol for another decade, during which time he continued to arrange for Sinatra and Cole, in addition to such Capitol artists as Dean Martin, Keely Smith, Sue Raney, and Ed Townsend. He also found time to release his own instrumental albums on the label, most notably "Hey...Let Yourself Go" (1957) and "C'mon...Get Happy" (1958), both of which peaked at a respectable number twenty on the
Billboard charts.