Thornton was selected by the Dallas Cowboys in the eighth round (219th overall) of the 1979 NFL Draft, after dropping because of his diminished production as a college senior. As a rookie, he helped to compensate the loss of Ed "Too Tall" Jones to his boxing retirement, by coming on passing downs in place of Larry Cole at left defensive end and unofficially finishing second on the team with 6 sacks. His best game came against the Chicago Bears when he registered 2 sacks, one pass defensed and a blocked extra point.