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, but was medically cleared to play during the Blues' Rookie Camp on September 5, 2018.
 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 their first conference final since 1996 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 suffered a wrist injury during Game 6 as the Blues beat the 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.
He was eventually medically cleared to play and was once again named to the Blues' opening night roster.
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.
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, but was medically cleared to play during the Blues' Rookie Camp on September 5, 2018.  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 their first conference final since 1996 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 suffered a wrist injury during Game 6 as the Blues beat the 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. He was eventually medically cleared to play and was once again named to the Blues' opening night roster. 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.
Robert Thomas (ice hockey)