Thornhill played college football at the University of Pittsburgh under legendary coach Glenn "Pop" Warner. An All-American offensive guard and tackle, Thornhill was given the ironic nickname "Tiny" due to his imposing size. Following his graduation from Pitt, Tiny became an assistant coach to Pop Warner but left midway through the season to play pro football with the Massillon Tigers, with teammates that included Knute Rockne, Jock Sutherland, Gus Dorais, Bob Higgins, and Bob Peck. He also played in the first-ever National Football League season in 1920 for the Cleveland Tigers and Buffalo All-Americans.