Thornton made his professional debut that season with the Low-ATri-City Valley Cats and spent the whole season there, going 4–0 with a 3.27 ERA in 15 games (12 starts). In 2016, he played for the High-ALancaster JetHawks and the Double-ACorpus Christi Hooks, compiling a combined 10–5 record with a 3.52 ERA in 24 games (21 starts), and in 2017, he pitched for Corpus Christi and the Triple-AFresno Grizzlies, posting a 9–6 record and a 5.21 ERA in 25 games (23 starts) between the two clubs.[7] He spent 2018 with Fresno. He was named the Pacific Coast League Pitcher of the Week for June 11–17 after pitching 72⁄3 scoreless innings in which he allowed only one hit (given up with two outs in the eighth inning) to go along with two walks.[8] In 24 games (22 starts) for the Grizzlies, Thornton went 9–8 with a 4.42 ERA.[9]
Toronto Blue Jays
On November 17, 2018, Houston traded Thornton to the Toronto Blue Jays for Aledmys Díaz.[10] The Blue Jays added him to their 40-man roster a few days later to protect him from the Rule 5 draft.[11] After participating in 2019 spring training, it was announced that Thornton had made the Opening Day roster on March 26.[12] He made his MLB debut on March 31, starting against the Detroit Tigers. He produced five shutout innings, and his eight strikeouts established a new franchise record for strikeouts in an MLB debut.[13] Thornton earned his first career win on May 14 at the San Francisco Giants, allowing two runs and striking out seven in 52⁄3 innings. He also had two hits and scored two runs in the interleague game, which Toronto won 7–3.[14]
With the 2020 Toronto Blue Jays, Thornton appeared in three games, compiling a 0–0 record with 11.12 ERA and six strikeouts in 52⁄3 innings pitched.[15] In 2021 with the Blue Jays, he pitched to a 1–3 record and 4.78 ERA with 52 strikeouts over 49 innings.[16] Thornton made 32 appearances for Toronto in 2022, pitching to an 0–2 record and 4.11 ERA with 37 strikeouts in 46.0 innings of work.
On January 13, 2023, Thornton signed a one-year, $1 million contract with the Blue Jays, avoiding salary arbitration.[17] Thornton was optioned to the Triple-A Buffalo Bisons to begin the 2023 season.[18] In only 4 games for Toronto, he logged a 1.69 ERA with 5 strikeouts in 5+1⁄3 innings of work. Thornton was designated for assignment on July 21, 2023, following the acquisition of Génesis Cabrera from the St. Louis Cardinals.[19]
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