Thornton studied at Portora Royal School and Stranmillis College, before becoming a teacher.
He also became the vice-president of the Fermanagh Young Unionist Association, and represented Northern Ireland in the Observer Mace debating competition.
[1] At the 1970 general election, Thornton stood for the Ulster Unionist Party (UUP) in Mid Ulster, but was narrowly defeated by Bernadette Devlin.
[1] He stood again at the February 1974 general election, in support of the Northern Ireland Assembly, but came bottom of the poll.
[2] He became a full-time organiser for the UUP, but, increasingly at odds with the direction of the party, he resigned in 1974 to join the newly founded Unionist Party of Northern Ireland.