Thornhill first game to hurling prominence with the Oldcastletown club while also making an impression as a schoolboy at St Colman's College.
He arrived on the inter-county scene as a member of the Cork minor hurling team in 1946, however, his two-year tenure in this grade ended without success.
He also earned selection for the Cork junior hurling team and was part of their All-Ireland JHC title triumph in 1947.
[2][3] Even though he had just turned 18, Thornhill was a member of the extended panel of the Cork senior hurling team that year.
Thornhill's religious studies impacted on his playing career and he made his last appearance in the Cork colours in a junior game in 1952.