He met Rosy McHargue, a well-known clarinetist, who took him to hear many leading musicians, including Bix Beiderbecke and Earl Hines.

He later moved to Los Angeles, where he worked with Henry Halstead, Earl Burtnett and Lennie Hayton, before joining Ben Pollack in 1934.

[6] Slack also recorded as an accompanist for Big Joe Turner, Johnny Mercer, Margaret Whiting and Lisa Morrow.

[1] In the original version of the song "Down the Road a Piece", recorded in 1940 by the Will Bradley Orchestra, Slack is mentioned in the lyrics: If you wanna' hear some boogie then I know the place It's just an old piano and a knocked-out bass.

His 1955 album, Boogie Woogie on the 88, featured a horn section including jazz musicians Shorty Sherock and Herbie Harper among others, with arrangements by Benny Carter.