Composer ("Cow Cow Boogie", "Strange Cargo", "Cuban Sugar Mill", "Riffette"), pianist and band leader, trained at the American Conservatory in Chicago. Originally trained as a drummer, Slack was active in the music business from 1927 and played piano for various bands, before and after moving to the West Coast. He was busy arranging for Ben Pollack and Jimmy Dorsey in the mid- to late 1930's, before making his mark as piano soloist in the short lived (1939-41) orchestra co-led by trombonist Will Bradley and drummer Ray McKinley. Slack was largely instrumental in changing the type of music the band played from ballads and regular swing to boogie-woogie and was featured on its biggest hit, "Beat Me Daddy Eight to the Bar".

In 1941, Slack founded his own orchestra, which attracted a strong following, especially when showcasing vocalist Ella Mae Morse. The band's biggest hit was "Cow Cow Boogie" and he had another with "Mr. Five by Five". A recording contract with the newly-created Capitol label (1942-47) ensured national exposure, as did regular appearances in films like Reveille with Beverly (1943), Follow the Boys (1944) and Seven Days Ashore (1944). After relinquishing his baton as orchestra leader, Slack remained based in California during the 1950's and 60's, albeit as part of a piano duo or trio, appearing in clubs and cocktail lounges. For the last few years of his life, he suffered from diabetes and was found dead in his Hollywood apartment in August 1965, from 'undetermined causes'.