Thornhill was a four-year starter at Michigan State, where he was awarded the Big Ten Medal of Honor in 2002.[1][2] After going undrafted in the 2002 NFL draft, Thornhill joined the Detroit Lions in training camp.[1] Thornhill was released on September 1, but rejoined the organization and was placed on the practice squad on September 3.[3][4] He was signed to an NFL deal and placed on the active roster on September 28.[5] Thornhill appeared in seven games that season.[6]
During the Lions' 2003 mini-camp, Thornhill suffered a stress fracture in his foot that left him unable to play for over a month.[7] He was released on July 23.[8]