He was selected by the Cleveland Browns in the 1st round (11th overall) of the 1984 NFL Supplemental Draft of USFL and CFL Players. During his rookie season with the Browns in 1985, he rushed for 1,104 yards and 7 touchdowns. Mack played for the Browns for nine seasons, from 1985 to 1993. In 1985 Mack rushed for 1,105 yards and halfback Earnest Byner rushed for 1,001 yards becoming just the third pair of teammates to run for over 1,000 yards for the same team in the same season. Nicknamed "Mack Truck", he was a two-time Pro Bowl selection in 1985 and 1987.