Born in La Crosse, Wisconsin, he learned to play drums as a boy. Later he took up xylophone and at 13 changed to piano. He studied with a local teacher throughout high school. At 17 his parents moved to Chicago where he continued his musical training. He met Rosy McHargue, a well known clarinetist, who took him to hear many leading musicians including Bix Beiderbecke and Earl Hines. His first job was with Johnny Tobin at the Beach View Gardens. He later moved to Los Angeles where he worked with Henry Halstead, Earl Burtnett and Lennie Hayton, before joining Ben Pollack in 1934.
He played with the Jimmy Dorsey Band in the 1930s and was a charter member of the Will Bradley Orchestra when it formed in 1939. Known to bandmates as "Daddy Slack," he played the piano solo on Bradley's recording of "Beat Me Daddy, Eight to the Bar", one of the early white boogie-woogie hits and a classic of the Big Band era.
After forming his own band in 1942 and signing with the newly founded Capitol Records, he recorded three songs at the third Capitol recording session on May 21, 1942. His recording of "Cow Cow Boogie," sung by the 17-year-old Ella Mae Morse, was the second record Capitol issued on July 1, and by July 25 it had reached number 1 on the hit parade. It was Capitol's firstgoldsingle.
T-Bone Walker was a member of Slack's band from 1942 to 1944 and Slack later accompanied Walker on his first solo recording for Capitol, Mean Old World. This band also had a hit with "Strange Cargo."
Slack continued to record with Capitol until at least 1950, recording some 80 tracks for the label.