Originally a Toronto Maple Leaf, Thomas was traded to the Los Angeles Kings but was later traded to the Montreal Canadiens. He spent roughly 3 years in the AHL with the Canadiens minor league affiliate, the Nova Scotia Voyageurs.
In 1972–73, in his first NHL pro start, Thomas recorded a 3–0 shutout versus the Vancouver Canucks on January 14, 1973. Thomas started a few games in goal right before the playoffs in order to give star netminder Ken Dryden a rest. Thomas did not dress for a playoff game. He was included on the Stanley Cup team picture, and given a Stanley Cup Ring. However, Thomas did not play enough games to qualify to be engraved on the Stanley Cup.
When Dryden decided to study law instead of tending goal in the 1973-74 NHL season Thomas played 42 games. However, Thomas was never really able to seriously contend for the starting goaltending position. In 1974–1975, Thomas was on the Canadiens' active roster for the entire season but did not play.
After his inactive 1974–75 campaign, Thomas was traded back to the Toronto Maple Leafs. This is where Thomas finally enjoyed some level of success. He won 28 games and was chosen to participate in the annual NHL All-Star game. This success was shortlived, because the following season Thomas struggled, and his position was overtaken by another Leaf goalie, Mike Palmateer.
He then became an assistant goaltending coach with the Rangers for three years. In 1985 he was named the coach of the Salt Lake Golden Eagles of the International Hockey League (IHL) and lasted two seasons. Thomas won the Commissioner Trophy as IHL coach of the year in 1987-88 as the Eagles won the Turner Cup.