He played for the Boston Bruins, San Jose Sharks, Toronto Maple Leafs and Florida Panthers of the National Hockey League (NHL).

[2] Thornton's on-ice vision, strength on the puck, deft passing ability and power forward style of play led to him becoming one of the league's premier centres and playmakers.

[11] Bruins head coach Pat Burns was measured in his deployment of Thornton, using him almost exclusively on the fourth line and making him a regular healthy scratch.

He played on a line with fellow NHL players Rick Nash and Niklas Hagman, helping HC Davos to win the League championship and the Spengler Cup.

[17] Boston's front office was apparently unhappy with Thornton's leadership style and for not raising his level of play during the playoffs.

[3] Thornton began the 2005–06 season strongly (33 points in 24 games), making him the team's leading scorer by a substantial margin, but the Bruins were struggling in the standings.

On November 30, 2005, Thornton was traded to the San Jose Sharks in exchange for forwards Marco Sturm, Wayne Primeau and defenceman Brad Stuart.

Upon arriving in San Jose, Thornton improved the Sharks' fortunes and found instant chemistry with winger Jonathan Cheechoo.

Due to Thornton's success, Cheechoo also enjoyed a career-season, winning the Rocket Richard Trophy as the NHL's top goal-scorer with 56 goals.

However, in the 2006 playoffs, Thornton was once again criticized for his play, as his production decreased to two goals and seven assists for nine points in 11 games as the Sharks were ousted in the second round by the eighth seeded Edmonton Oilers.

With a league-leading 92 assists, Thornton became only the third player in NHL history to record back-to-back 90-assist seasons, joining Wayne Gretzky and Mario Lemieux.

However, Thornton was effectively neutralized by Red Wings defenceman Nicklas Lidström,[23] for the remainder of the series as the Sharks were eliminated in six games.

In the subsequent playoffs, he recorded a goal and four assists in six games as the Sharks were upset in the first round by the eighth seeded Anaheim Ducks.

[26] In September 2009, before the start of the 2009–10 season, the Sharks acquired Dany Heatley in a three-player trade that sent Thornton's struggling former linemate Jonathan Cheechoo, left-winger Milan Michálek and a second-round draft pick to the Ottawa Senators.

Although the line's production slowed down in the second half of the season, all three Sharks players finished in the League's top 15 in point-scoring.

The Sharks entered the 2010 playoffs as the top seed in the Western Conference for the second-consecutive year and the Presidents' Trophy runner-up behind the Washington Capitals.

After the elimination, team management vacated all the Sharks' captaincy positions, including Thornton's role as one of the alternate captains.

Prior to the 2010–11 season, he was chosen to replace the retiring Rob Blake as the eighth captain in team history on October 7, 2010.

Near the start of the 2010–11 season, Thornton scored the fourth hat-trick of his NHL career against Martin Brodeur in a 5–2 win over the New Jersey Devils.

On November 6, 2010, Thornton was suspended two games for a controversial hit to the head against St. Louis Blues forward David Perron.

[31] Thornton finished the 2013–14 season with 11 goals and 65 assists as the Sharks amassed 111 points, just six short of their franchise's all-time-high mark, and were among the favourites to win the Stanley Cup.

In the 2016 playoffs, the Sharks beat the Los Angeles Kings in the first round in five games, avenging a previous loss to them two years earlier.

[40] On February 11, 2019, in a 7–2 win over the Vancouver Canucks, Thornton passed Gordie Howe for ninth place on the NHL all-time assists list.

[41] Thornton and the Sharks reached the Western Conference Finals, though they were eliminated by the eventual Stanley Cup champion St. Louis Blues in six games.

[45] With the 2020–21 season delayed due to the ongoing COVID-19 pandemic, Thornton returned to HC Davos of the National League on October 15, 2020, for a third stint.

[53] At the time of his retirement, Thornton was 12th in league history in total points scored, seventh in assists, and sixth in games played.

At the 2005 IIHF World Championship in Austria, Thornton led all scorers with 16 points (six goals and ten assists) in nine games and was named tournament MVP.

Thornton was later invited to the Canada's hockey camp for the 2014 Winter Olympics, but did not attend due to his son being hospitalized with an illness.

[66] Born in St. Thomas, Ontario, Thornton became a naturalized American citizen in July 2009 at a ceremony in Campbell, California, a suburb of San Jose;[65] he later also received a Swiss passport.

[67][68] The Tragically Hip lead vocalist Gord Downie's song "You Me and the B's" (from his 2017 solo album Introduce Yerself) includes a lament about Thornton's poorly-received trade from the Bruins to the Sharks in 2005.