Thornton was a second-round selection by the Steelers in 1977 out of Northwestern State, a small school in Shreveport, Louisiana. The New Orleans native played all six of his NFL seasons with the Black and Gold, and was a member of the Super Bowl XIII and XIV championship teams in 1979 and 1980. Thornton gained 86 total yards and scored his first postseason touchdown against the Miami Dolphins in the 1979 divisional game. He also caught a 22-yard pass in Super Bowl XIV against the Los Angeles Rams.