He made his MLB debut with the team in 2019, but struggled with injuries and was unable to find consistent playing time before he was traded to the Nationals during the 2021 season, with whom he became their starting center fielder.

In 2015, he played for both the Vancouver Canadians[7] and the Lansing Lugnuts, hitting a combined .206 with five home runs and 35 RBIs in 52 total games.

He spent 2016 with Lansing where he compiled a .216 batting average with seven home runs, 27 RBIs, and 17 stolen bases in 81 games.

[9] On July 2, 2017, Toronto traded Thomas to the St. Louis Cardinals in exchange for international signing bonus cap space.

[19] On April 19, against the New York Mets at Busch Stadium, he hit a home run in his first major league at bat.

On July 30, 2021, Thomas was traded to the Washington Nationals in exchange for starting pitcher Jon Lester.

[30] In 2022 with the Nationals, he played in 146 games, led the majors in percentage of balls hit safely (23.2%), and batted .241/.301/.404 with 17 home runs, 52 RBIs, and 26 doubles.

[32] In 2023, Thomas became the Nationals primary right fielder and enjoyed a breakout season, batting .268 and setting career highs with 28 home runs and 20 stolen bases.

[33] On January 11th, 2024, Thomas and the Nationals agreed to a one-year contract worth $5.45 million to avoid salary arbitration.

[34] On July 29, 2024, Thomas was traded to the Cleveland Guardians in exchange for Alex Clemmey, José Tena, and Rafael Ramírez Jr.[35] On October 12, in Game 5 of the American League Division Series against the Detroit Tigers, Thomas hit a grand slam off Tarik Skubal in the bottom of the fifth for the Guardians to take the lead at 5–1 and propel them to the ALCS for the first time since 2016 with a 7–3 win.