Fellow seamer Oliver Hannon-Dalby, 28, and batsman Matthew Lamb, 20, have both signed one-year extensions, to the end of the 2018 season.
Thornton, 24, made his first-team debut for the Bears in the One-Day Cup win over Northamptonshire on 27 April.
A week later, he signed a three-month deal - his first professional contract.
Thornton, who is currently sidelined through injury, helped Berkswell to win the Birmingham League title last summer, having been a Bears second XI regular for the last two seasons.
Since being drafted into the Bears side for his debut, he has played three more One-Day games, finishing on the winning side in two of them.
He also took 4-34 against Somerset on his first-class debut at Taunton - the first of two Championship appearances so far.
Lamb, who turns 21 next week made his first-class debut at Taunton last season, and scored 51 in the first innings of last week's one-wicket defeat by champions Middlesex at Edgbaston, having been brought in for his first appearance of the season to replace injured captain Ian Bell.
Hannon-Dalby, who joined Warwickshire from his native Yorkshire in 2013, has made just two Championship appearances in 2017, having been dropped following the Bears' poor start to the season.
But he played in five of this season's One-Day Cup games, having been part of the side that won the trophy against Surrey at Lord's last September.
He was also in the team when the Birmingham Bears won the T20 Blast in 2014 - and has so far taken five wickets in two wins out of two in this season's group games.
Warwickshire fast bowler Grant Thornton has signed an extended contract with the Bears until the end of the season as a reward for his promising form.