Thornton played on the junior varsity team as linebacker in 1974 and intercepted one pass. In 1975, he made the switch from linebacker to defensive end, listed behind Andy Peters and Joe Saunders. In 1976, Thornton was listed as number one defensive end on the left side. He started three games in this year and played in all 11, recording 28 tackles. In 1977, he was listed as the number two defensive end behind Fran Gleason. He intercepted one pass in the Gold-Blue Game that spring. He majored in business. He played football at Parkersburg High School.