Ottawa Senators
Thomson was selected by the Ottawa Senators of the National Hockey League (NHL) in the first round, 19th overall, of the 2019 NHL entry draft. On 15 July 2019, Thomson was signed to a three-year, entry-level contract with the Senators. He joined Ottawa's American Hockey League (AHL) affiliate, the Belleville Senators, during the 2020–21 season and made 35 appearances, tallying one goal and 13 points.
Thomson was invited to the Senators training camp ahead of the 2021–22 season. On 3 October 2021, he was assigned to AHL. Thomson made his NHL debut on 11 November, 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. He recorded his first NHL point in the following game on 13 November on an assist in a 6–3 victory over the Pittsburgh Penguins. He was returned to Belleville on 8 December, after playing in eleven games. He was recalled for one game on 1 January 2022, against the Toronto Maple Leafs and then called up again on 25 January. He finished the season playing in 16 games for the Senators, tallying five points (all assists). In 44 games with Belleville, he added ten goals and 26 points. Belleville made the playoffs and faced the Rochester Americans in the opening round. However, the Americans eliminated them and in the two games in the series, Thomson was held scoreless. At the beginning of the 2022–23 season, Thomson was assigned to Belleville. He was recalled by Ottawa on 22 November 2022. He was immediately slotted into the roster alongside Nick Holden in the 24 November game versus the Vegas Golden Knights, giving up a goal in the third period. He played one more game before being sent back to Belleville. While with Belleville, Thomson suffered an upper-body injury in December that kept him out for 11 games. He finished the season with seven goals and 33 points in 56 games with Belleville.
On 1 October 2023, after being put on waivers by the Senators, Thomson was claimed by the Anaheim Ducks. However, Anaheim waived Thomson again one week later, and he was subsequently reclaimed by the Senators. He was then assigned to Belleville after no other team put in a claim. Thomson played out the 2023–24 season in Belleville, registering six goals and 21 points through 67 regular season games. 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. In five playoff games, he added two assists.
Malmö Redhawks and return to Ottawa
As a pending restricted free agent with the Senators, Thomson opted to return to Europe and was signed to a two-year contract with Swedish club, Malmö Redhawks of the Swedish Hockey League (SHL), on 13 May 2024. In 50 games with Malmö, Thomson scored 17 goals and 29 points. The Redhawks made the SHL playoffs and in eight playoff games, he marked three goals and four points.
On 17 June 2025, Thomson was released from the final year of his contract in the SHL and re-signed by the Ottawa Senators on a one-year, two-way contract. He attended Ottawa's training camp and was one of the last cuts by Ottawa at the end of camp. He was placed on waivers in October by the Senators for the purpose of assigning him to Belleville. He started the 2025–26 season with Belleville after going unclaimed but was recalled by Ottawa on 19 November after defenceman Thomas Chabot suffered an injury and the team was going on a seven-game West Coast road trip. He did not play however, and was returned to Belleville on 22 November. On 14 March 2026, he was recalled again, but in his first appearance after being called up on 24 March against the New York Rangers, he was injured in the second period. He returned to the lineup on 28 March and registered his first point of the season on 31 March, assisting on a goal by Michael Amadio in a 6–3 loss to the Florida Panthers. He finished the season with three assists in 11 games with Ottawa and 14 goals and 25 points in 55 appearances with Belleville. The Senators made the playoffs, and faced the Carolina Hurricanes in the opening round. Thomson did not play in the first game, but replaced the injured Artem Zub in game two, making his NHL playoff debut on 20 April. However, the Senators were swept in the first round by the Hurricanes. Thomson went scoreless in his only game.
