A long-time member of the Buttevant club, he was also a four-time All-Ireland Championship winner with the Cork senior hurling team.
He made his debut on 9 July 1933 when he lined out at full-back in Cork's 7-05 to 4-05 defeat of Clare but was left out of the team for the subsequent Munster final.
Thornhill was recalled to the team for the 1934 Munster final against Waterford, but ended the game on the losing side after the 7-10 to 5-02 defeat.
After a number of years of no association with the team, Thornhill was restored to the full-back berth for the 1937 Munster Junior Championship.
He was again selected at full-back when Cork faced London in the All-Ireland final and claimed a winner's medal after the 4-04 to 4-01 victory.
Thornhill's performances at junior level brought him to the attention of the senior selectors and he was drafted onto the team during the 1938-39 National League.
On 3 September 1939, Thornhill lined out at full-back when Cork suffered a 2-07 to 3-03 defeat by Kilkenny in the "thunder and Lightning" All-Ireland final.
[2] Thornhill ended the season by lining out in Cork's 5-04 to 2-05 defeat by Tipperary in the delayed Munster final.
[3] Thornhill lined out in a fourth successive Munster final the following year, with Cork claiming the title after a 4-15 to 4-01 defeat of Tipperary.
It was the first of three successive winners' medals for Thornhill, as he was also included on the team for the final victories over Leinster in 1943 and Connacht in 1944.
In retirement from playing, Thornhill remained heavily involved in the administration and organisation of the Buttevant club.