[5] As he grew out of Bantam hockey, Thomas played for the St. Andrew's College Saints under-16 team for two years while also attending the private school.

[11] As a result, he was named to the 2017 CHL/NHL Top Prospects Game alongside teammate Alex Formenton[12] and was the recipient of the OHL's Most Sportsmanlike Player of the Year Award.

[13] His offensive play also earned him a boost in ranking from 28th to 22nd amongst North American skaters by the NHL Central Scouting Bureau.

[16] Following an impressive training camp and pre-season with the Blues, Thomas was signed to a three-year, entry-level contract on September 28, 2017, before he was returned to continue his development with the London Knights for the 2017–18 season.

[20] His run as captain was short-lived, however, as on January 8, 2018, Thomas was traded to the Hamilton Bulldogs in exchange for Connor McMichael and five draft picks.

[21] While with the Bulldogs, Thomas was suspended five games for slashing Peterborough Petes forward Zach Gallant in the mouth, though he still finished the regular season tied for 20th around the league in scoring.

[22] He continued his scoring success during the 2018 OHL Playoffs and recorded 32 points in 21 games to help the team clinch the J. Ross Robertson Cup.

[23] Upon the conclusion of the 2018 Memorial Cup Semifinal game, Thomas revealed he suffered an ankle injury during the Bulldogs series against the Sault Ste.

[30] Thomas remained on the team's roster for the entirety of the season and finished with 33 points in 70 games as the Blues qualified for the 2019 Stanley Cup playoffs.

[32] Afterwards, former Blues player Keith Tkachuk said, "For someone that young, to clearly stand out in a Game 7 from start to finish, was one of the best performances you have ever seen in the National Hockey League, in my opinion.

[39] Although the NHL suspended play due to COVID-19, Thomas placed fifth on the team in goals per game and first with primary assists.

[40] During the pause in play, Thomas captained a league-wide Fortnite tournament for charity with teammates Vince Dunn and Jordan Kyrou.

[42] Thomas returned to the Blues' lineup for their game against the Golden Knights on March 22, 2021, over a month after his initial injury.

[50] Playing alongside fellow Blues prospect Jordan Kyrou, Thomas helped Canada clinch a gold medal after a 3–1 win against the Sweden men's national junior ice hockey team.