[7] The following year, he became the tenth goalie to record a shutout in his NHL debut, with a 3–0 Canadiens victory against the Vancouver Canucks.
[9] In 1973–74, with Dryden sitting out the season to complete his bar exam, he shared duties with Plasse and Michel Larocque, playing 42 games and sporting a 2.76 goals against average,[10] but again was not used in the playoffs.
[12] He then spent the entire 1974–75 season as Montreal's number 3 goalie, but did not see action in a single game as Dryden returned with Larocque as his backup.
[10][12] Thomas was frustrated, but appeased when Montreal general manager Sam Pollock advised him that the team intended to not utilize him but would trade him to a "good situation" in the off season.
[3] After two years in Toronto, he became expendable upon the emergence of Mike Palmateer, and the New York Rangers acquired him in the 1977 NHL Waiver Draft, to use him as the backup to John Davidson.
In 1985 Thomas became the head coach of the Salt Lake Golden Eagles of the International Hockey League (IHL), a Rangers affiliate.
[20] In 1986, he returned to the NHL as assistant coach of the Chicago Blackhawks,[3] a hiring based in part on his work with goalies such as Mike Vernon with the Golden Eagles, and Glen Hanlon and John Vanbiesbrouck with the Rangers.
[21] Under head coach Bob Murdoch, Thomas focused on Chicago's goalies and was involved in the development of Jimmy Waite and Ed Belfour.