He represented Stanstead in the Legislative Assembly of Quebec from 1875 to 1878 and from 1881 to 1886 as a Conservative. He was born in Derby, Vermont, the son of John L. Thornton and Dolly Bagley, and was educated there and at Stanstead College. He came to Lower Canada in 1841 and set up business at Barnston, later moving to Coaticook.
Thornton sold equipment used in the construction of the railway between Richmond and Stanhope.
He served on the council for Barnston township, was mayor of Coaticook in 1872 and 1873 and was warden for Stanstead County from 1872 to 1874. He was defeated by Henry Lovell when he ran for reelection in 1878.
Thornton died at Coaticook at the age of 64.