Thornton was a standout for the Scotland Under-20 team between 2016-2018, making 17 appearances for the representative side. His performances with the team earned him an academy contract with the London Wasps in 2017. He made four appearances for the Premiership side before moving to Nottingham, in the RFU championship, on loan in 2019.
After a short stint with Nottingham, the prop signed with the Glasgow Warriors on a two-year deal. He was with the Warriors until the end of the 2021 season when he signed on with the Ayrshire Bulls, a Scotland Super Six club.
Thornton made the move to the Shute Shield after a year with the Bulls. Strong performances in the competition garnered attention from numerous Super Rugby Clubs. In the end, the loosehead opted to sign on with the Chicago Hounds, bolstering the club’s front row just two weeks before the regular season begins.