Thomas, 6-foot-2, 180 pounds, began the 2015-16 season with the Calgary Hitmen of the Western Hockey League. He recorded three assists in 14 games with Calgary prior to being traded to the Vancouver Giants, also of the WHL, on October 27. Thomas skated in 60 games with the Giants, registering eight goals and 25 points with 40 penalty minutes. He led all Vancouver defensemen for goals and ranked second for assists and points.

The Calgary native has appeared in 213 career WHL games with the Hitmen and Giants, recording 22 goals and 90 points. He also played for Team Canada at the 2014 Under-18 World Championships, tallying one assist in seven games en route to earning the Bronze Medal.

Thomas was drafted by the Lightning in the fourth round, 119th overall, of the 2014 NHL Draft.