Williams was selected by the Dallas Cowboys in the seventh round (208th overall) of the 1976 NFL draft as an athlete, with the intention of playing him at running back, but he opted not to sign with the team.
On May 19, 1976, he was signed by the Calgary Stampeders of the Canadian Football League to play quarterback.
The next year, he was used mostly on special teams as a kickoff returner, then was released by new head coach Bill Walsh on August 20, 1979.
He started twelve games at fullback after Roland Harper was lost for the year with a knee injury, while blocking for Walter Payton and registering 755 all-purpose yards.
The biggest highlight of Williams' NFL career came on Thanksgiving Day in 1980 against the Detroit Lions, when he returned the opening overtime kickoff 95 yards for the game-winning touchdown, completing a comeback from a 17–3 fourth-quarter deficit.