In the 1977 NFL draft, Thornton was selected by the Pittsburgh Steelers in the second round as the 48th pick. He played six seasons with the team from 1976 to 1982, appearing in 74 games, and accumulating 1,512 yards, 18 rushing touchdowns, 46 catches, and 515 receiving yards, along with six touchdown receptions. He was part of the team's Super Bowl-winning seasons in 1978 and 1979, serving as a backup to Franco Harris and Rocky Bleier.
Thornton had a standout game against the Miami Dolphins in the divisional round of the 1979 playoffs, where he ran for 52 yards and a touchdown, leading the team to a 34-14 victory and a Vince Lombardi Trophy. After two years of absence from football, he played for one season with the Oklahoma Outlaws in the United States Football League.