 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.He made his championship debut on 25 June 1939 in a 7-04 to 4-03 defeat of Waterford before later claiming his first Munster Championship medal after a 4-03 to 3-04 victory over Limerick in the final.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.