He was drafted 19th overall by the Ottawa Senators in the first round of the 2019 NHL entry draft.[1][2][3] On July 15, 2019, Thomson was signed to a three-year, entry-level contract with the Senators.[4]
Thomson was invited to the Senators training camp ahead of the 2021–22 season. On October 3, 2021, he was assigned to the Senators American Hockey League (AHL) affiliate, the Belleville Senators.[5] Thomson made his NHL debut on November 12, 2021, with the Senators in a 2–0 loss to the Los Angeles Kings after being recalled the same day from Belleville due to a breakout of COVID-19 in the team.[6] He was returned to Belleville on December 8, 2021, after playing in eleven games.[7] He was recalled for one game on January 1, 2022, against the Toronto Maple Leafs and then called up again on January 25, 2022.[8] He finished the season playing in 16 games for the Senators.[9] At the beginning of the 2022–23 season, Thomson was assigned to Belleville.[10] He was recalled by Ottawa on November 22, 2022.[11] He was immediately slotted into the roster alongside Nick Holden in the November 24 game versus the Vegas Golden Knights, giving up a goal in the third period.[12] He played one more game before being sent back to Belleville.[13][14] While with Belleville, Thomson suffered an upper-body injury in December that kept him out for 11 games.[15]
On October 1, 2023, after being put on waivers by the Senators, Thomson was claimed by the Anaheim Ducks.[16] However, Anaheim waived Thomson again one week later, and he was subsequently reclaimed by the Senators.[17] He was then assigned to Belleville after no other team put in a claim.[18] Thomson played out the 2023–24 season in Belleville, registering 6 goals and 21 points through 67 regular season games.[19] He helped Belleville make it to the second round of the Calder Cup playoffs for the first time in team history, but was surpassed by other prospects.[20]
