A former minor pro hockey player who toiled in the Montreal Canadiens' organization, Slack signed with the Red Sox in 1952, and won 15 games in his rookie season for the Roanoke Red Sox of the Class B Piedmont League.
He reached the highest minor league level with the San Francisco Seals and the Seattle Rainiers of the Pacific Coast League and the Louisville Colonels, Indianapolis Indians and Minneapolis Millers of the American Association, but pitched in only 70 games over parts of five seasons with those teams.