On his return from the war, Mooney became a freelance arranger in Hollywood and started to make a name for himself, providing charts for top vocalists such as Bing Crosby, Frank Sinatra, Judy Garland, Peggy Lee, Kay Starr, and Billy Eckstine. His well-known arrangement of Kay Starr's hit, "Wheel of Fortune" in the early 1950s, led to his contract with Mercury Records in New York City.
Mooney was described as a "nuts and bolts arranger" by fellow chart writer Billy May. It was meant as a compliment, and Mooney's versatility can be characterised by some of the more notable arrangements he wrote at the Mercury stable. For Vaughan, he provided lush, string settings (along with more brassy, upbeat numbers) on albums such as Great Songs From Hit Shows (1956) and Sarah Vaughan Sings George Gershwin (1957). Though those arrangements grated with some jazz purists, their dramatic qualities inspired the acclaimed singer to some of her finest work, for example Hit Shows' "It Never Entered My Mind" and the Gershwin album's Isn't It a Pity?. Apart from such highbrow projects, Mooney also arranged some of Vaughan's forays into R&B-inspired '50s pop, for example the Clyde Otis-penned "Sweet Affection" (1957).
Mooney's numerous arrangements for Dinah Washington, meanwhile, ranged from straight big bandswing, through Latin mambo to rhythm and blues, but the seven albums on which he worked with Simone, between 1964 and 1967, display the greatest versatility. Though they contain the occasional string-backed track not unlike Mooney's classic work with Vaughan, they took him a world away from it at other times. For example, Simone's final Philips album, High Priestess of Soul (1967), is a diverse combination of pop, jazz and gospel. Mooney's two best-known arrangements for Simone, "I Put A Spell On You" and "Feelin' Good", might be quite different in some ways to his earlier work with Vaughan, but the same sense of drama is evident.
In his A&R capacity, Mooney helped to woo Xavier Cugat to Mercury, and he also produced several albums, including Dizzy Gillespie's Jambo Caribe (1964) and Vaughan's penultimate work for Mercury It's a Man's World (1967), for which Mooney also arranged some of the songs.
Having joined ASCAP in 1936, Mooney's compositions included "Rigamarole", "Hodge-Podge", "Jumpin' Jiminy", "Sing, It's Good For Ya", and "Goin' to Town".
