 An All-Ireland medal winner at junior level, Thornhill was drafted onto the Cork senior team in 1939.From his debut, he quickly made the full-back position his own and made 22 championship appearances in a career that ended with his last game in 1944.During that time Thornhill was part of four All-Ireland Championship-winning teams as part of Cork's record-breaking four titles in-a-row between 1941 and 1944.