Premier Rodmond Roblin's Conservatives won the election, and Thornton served as an opposition member for the next three years.
The Roblin government was re-elected, but was forced to resign in 1915 after being found responsible for corruption in the tendering of grants for new legislative buildings.
The Liberals, though holding a minority of seats, were permitted to form government, and Thornton was appointed as Minister of Education on 12 May 1915.
The Liberals were reduced to a minority government in the 1920 provincial election, and were weakened by the rise of organized Farmer and Labour political groups.
He was defeated in the 1922 provincial election, finishing third against United Farmers of Manitoba candidate Duncan Stuart McLeod.