Gaillard first rose to prominence in the late 1930s as part of
Slim & Slam, a jazz novelty act he formed with bassist
Slam Stewart. Gaillard, with
Dodo Marmarosa on piano, appeared as a guest several times on
Command Performance, recorded at KNX radio studios in Hollywood in the 1940s and distributed on transcription discs to American troops during World War II. '' In December 1942, Gaillard was drafted into the
United States Army Air Forces. There, Corporal Gaillard was an assistant crew chief working on
Martin B-26 Marauder bombers at
Laughlin Army Air Field in
Del Rio, Texas. He was discharged before the end of the war after a year and seven months of service, six months after being diagnosed with chronic frontal sinusitis. He resumed his music career on his release from the draft in 1944. Upon his return he released the song
"Atomic Cocktail", which featured seemingly lighthearted lyrics laced with symbolism about nuclear war. Gaillard later teamed with bassist Bam Brown, and their successes included the
hipster anthem "Opera in Vout (Groove Juice Symphony)". They can be seen in ''O'Voutie O'Rooney'', a 1947 motion picture featurette filmed live at one of their nightclub performances. Slim and Bam were featured at the first Cavalcade of Jazz concert at
Wrigley Field in Los Angeles produced by
Leon Hefflin Sr. on September 23, 1945, along with
Count Basie. Gaillard also played for the 2nd Cavalcade of Jazz at Wrigley Field on October 12, 1946, and for the 3rd Cavalcade of Jazz, also at Wrigley Field on September 7, 1947. In the late 1940s and early 1950s, Gaillard frequently opened at
Birdland for
Charlie Parker,
Flip Phillips, and
Coleman Hawkins. His December 1945 session with Parker and
Dizzy Gillespie is notable, both musically and for its relaxed convivial air. "Slim's Jam", from that session, is one of the earliest known recordings of Parker's speaking voice. In 1949, Gaillard was playing in San Francisco. An account of meeting Gaillard at a performance there can be found near the end of Part Two of
On the Road. Gaillard could play several instruments and manage to turn the performance from jazz to comedy. He would play guitar with his left hand fretting with fingers pointing down over the fingerboard (instead of the usual way up from under it), or would play credible piano solos with palms facing up. Gaillard wrote the theme for the Peter Potter radio show, and wrote and recorded the "Don Pitts On the Air" theme in 1950 for San Francisco DJ Don Pitts. On March 27, 2008, the Pitts theme entered the archives of the Rock and Roll Hall of Fame in
Cleveland, Ohio. In the early 1960s, Gaillard lived in
San Diego,
California. During that time, he recorded several singles and performed with local bands. Under the name Slim Delgado, he recorded "Frank Rhoads Round", a rock-and-roll single for the Xavier label. On the B-side is a song called "Dr. Free". Gaillard appeared on several TV shows during the 1960s and 1970s, including
Marcus Welby, M.D., ''
Charlie's Angels, Mission: Impossible, Medical Center, The Flip Wilson Show, and Then Came Bronson. He also appeared in the 1970s TV series Roots: The Next Generations and reprised some of his old hits on the NBC prime-time variety program The Chuck Barris Rah Rah Show''. By the early 1980s, Gaillard was touring the European jazz festival circuit, playing with such musicians as
Arnett Cobb. He also performed with
George Melly and
John Chilton's Feetwarmers, appearing on their BBC television series and occasionally deputising for Melly when he was unwell. Gaillard's onstage behavior was often erratic and nerve-wracking for the accompanying musicians. He made a guest appearance on Show 106 of the 1980s program
Night Music, an NBC late-night music series hosted by
David Sanborn. Gaillard followed Dizzy Gillespie's advice to move to Europe and settled in
London in 1983. Around Christmas 1985, Gaillard recorded the album
Siboney at Gateway Studios in
Battersea, London, produced by
Joe Massot. As Massot recalled later: {{quote box|border=none|fontsize=90%|quote= I was introduced as Cuban. "Rooney! I am Cuban too." [...] Slim said how much he wanted to make a Latin record and talked about his friend the great Cuban leader Machito. This was 1985. I had been flying between Angola, where Cuban troops were fighting and Miami where one million exiled Cubans live. We talked of all those Cubans who wanted to go back to Cuba but couldn't. [...] In his inimitable, enthusiastic way Slim was trying to sell me, a movie maker, the idea of recording an album. ==Death==