Thornhill first played for the Buttevant club at adult level in the late 1920s. A decade later he had assumed the captaincy of the Buttevant intermediate team and led the team to a meeting with Ballincollig in the 1940 Intermediate Championship final. Thornhill scored a goal from a 70-yard free and ended the game with a winners' medal after the 2-04 to 1-04 victory. He retired from club duty in 1952, having also lined out with divisional side Avondhu in the Senior Championship.
