Over eight seasons in Vancouver, Gradin established himself as the team's all-time leading scorer and competed in the 1982 Stanley Cup Finals, as well as the 1985 NHL All-Star Game.
After a one-year stint with the Boston Bruins, Gradin returned to Sweden, where he finished off his playing career with his former SEL team, AIK.
After scoring 16 goals and 39 points in 35 games in the 1975–76 SEL season, Gradin was selected by the Chicago Black Hawks in the third round, 45th overall, of the 1976 NHL Amateur Draft.
In June 1978, the Black Hawks traded Gradin's NHL rights to the Vancouver Canucks in exchange for a draft pick.
Playing on a line with fellow rookies Stan Smyl and Curt Fraser, Gradin led the team in assists (31) and finished second in team-scoring (51 points) in 1978–79.
His efforts earned him the Cyclone Taylor Award as team MVP, an honour he shared with goaltender Glen Hanlon.
Gradin continued to play a starring role for the team in the 1982 playoffs, leading the Canucks in post-season scoring with 19 points in 17 games.
Despite entering the playoffs with a losing record (30 wins, 33 losses and 17 ties), the Canucks went on to their first Stanley Cup Finals appearance in franchise history.
Gradin's 19 points ranked fourth in league scoring, behind the Islanders trio of Brian Trottier, Mike Bossy and Denis Potvin.
His tenure as the franchise's scoring leader was short-lived, as longtime teammate Smyl surpassed his career mark during the ensuing season.
Four years after his retirement as a player, Gradin returned to the Vancouver Canucks organization as an amateur scout prior to the 1994–95 NHL season.
During that time, he played a key role in the Canucks drafting Daniel and Henrik (1999), Mattias Öhlund and Alexander Edler.
Peter Gradin, Thomas' younger brother, was also a star hockey player in Sweden throughout the 1980s although he was never drafted by an NHL team.