Youth Growing up in Aurora, Thomas played
Bantam and AAA ice hockey with the York Simcoe Express of the Eastern AAA Minor Midget Hockey League. During the 2014–15 season, Thomas recorded 45 points in 34 games for the Express. 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. His younger brother would also attend and graduate from the school in 2019.
Major junior Thomas was selected by the
London Knights of the OHL in the second round, 26th overall, in the 2015
Ontario Hockey League (OHL) Priority Selection. Knights coach Dale Hunter praised Thomas for his "vision, speed, hockey sense and ability to pass the puck" and considered him a replacement for former Knight
Bo Horvat. In his rookie season with the Knights, Thomas recorded 15 points in 40 games as they qualified for the
2016 OHL Playoffs. and won the
2016 Memorial Cup. Thomas played his rookie season as a defensive forward, which he developed into a more offensive role prior to his sophomore season. Thomas developed into a more offensive role with the Knights during the
2016–17 season and recorded a career high 66 points in 66 games. and was the recipient of the OHL's Most Sportsmanlike Player of the Year Award. His offensive play also earned him a boost in ranking from 28th to 22nd amongst North American skaters by the
NHL Central Scouting Bureau. Thomas shared his surprise at his jump in ranking by saying, "I actually was expecting, from talking to people, to go in the third round...I was talking to a lot of (U.S. schools) at the time, and when I jumped to the second, I was pretty shocked." He was eventually chosen in the first round, 20th overall, by the
St. Louis Blues and invited to their rookie camp over the summer. 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. As the Knights were waiting on key players to return from their respective NHL clubs, the Knights began the season with a rocky start and a 1-4-0 record. On November 3, 2017, Thomas was named captain of the Knights, alongside alternate captains
Sam Miletic, Max Jones,
Cliff Pu, and
Evan Bouchard. Shortly thereafter, he recorded the game-winning goal to clinch coach Dale Hunter his 700th regular season win. 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. 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. 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. As a result of his success, Thomas was awarded the
Wayne Gretzky 99 Award as MVP of the playoffs.
Professional 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. Marie Greyhounds. As a result, Thomas was unable to attend the Blues' development camp, Thomas eventually made the Blues' opening night roster for the
2018–19 season, and subsequently made his NHL debut on October 4 against the
Winnipeg Jets. However, during the following six games, Thomas was a healthy scratch in the Blues' lineup for two of them. Working his way back into the lineup, Thomas recorded his first NHL point on October 12 in a 5–3 win over the
Calgary Flames, and his first NHL goal on November 21 in a 4–1 loss to the
Nashville Predators. 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. At the age of 19, Thomas helped the Blues advance to conference final by assisting on both goals during Game 7 of the Western Conference Second Round. 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." As the youngest player competing during the Western Conference Final against the
San Jose Sharks, He returned to the lineup for Game 1 of the
2019 Stanley Cup Finals against the
Boston Bruins, but suffered another injury and was out of the lineup until Game 6. His efforts during the postseason helped the Blues win their first Stanley Cup in franchise history. Upon winning the Cup, Thomas' hometown of Aurora added a sign underneath their city's welcome sign stating "Home of Robert Thomas, Stanley Cup champion." Prior to the
2019–20 season, Thomas underwent surgery to repair a tendon in his left wrist. Leading up to opening night, Thomas was expected to have a breakout season and a "Smooth sophomore season" in the top six. Although the NHL suspended play due to
COVID-19, Thomas placed fifth on the team in goals per game and first with primary assists. During the pause in play, Thomas captained a league-wide
Fortnite tournament for charity with teammates
Vince Dunn and
Jordan Kyrou. Together, they finished in second place and donated $50,000 to the
St. Louis Children's Hospital, $25,000 to the St. Louis Area Foodbank, and $25,000 to
muscular dystrophy research. Thomas began the shortened
2020–21 season strong by tallying one goal and five assists for six points through the first 12 games of the season. However, he suffered a thumb injury on February 6, 2021, and was expected to be sidelined for 4–6 weeks. Thomas returned to the Blues' lineup for their game against the Golden Knights on March 22, 2021, over a month after his initial injury. He was a minus-1 in 13:26 minutes of ice time as the Blues fell 5–1. However, his return was shortlived as he suffered a lower body injury during a game against the
Minnesota Wild on April 10. He subsequently missed four games before rejoining the Blues on April 23. Thomas signed a two-year, $5.6 million contract extension with the Blues on September 21, 2021. On July 13, 2022, Thomas signed an eight-year, $65 million extension with the Blues. ==International play==