Widely considered at one time to be one of the best defencemen in the NHL, Hedman is a six-time James Norris Memorial Trophy finalist, winning the award in 2018.

Following his performance at the 2009 World Junior Championships for Sweden, Hedman was listed atop the rankings of European skaters by the NHL Central Scouting Bureau.

Despite the unlikelihood of Hedman playing in Russia, he was drafted 83rd overall by the Kontinental Hockey League (KHL)'s Spartak Moscow on 1 June 2009.

On 5 January 2011, halfway into his sophomore NHL season in 2010–11, Hedman checked Pittsburgh Penguins captain Sidney Crosby from behind, receiving a minor boarding penalty.

[13] In January 2013, Hedman returned to the NHL after lockout ended, playing 44 games of the shortened 48-game season and registering 4 goals, 16 assists and 20 points.

However, his strong start would be derailed after he suffered a broken finger in a game against the Vancouver Canucks on 18 October 2014, which was expected to rule him out of play for four-to-six weeks.

[15] At the time of this injury, Hedman was tied for the lead amongst NHL defencemen with seven points and was considered a possible James Norris Memorial Trophy candidate for Defenceman of the Year entering into the season.

[19] On 20 May 2016, Hedman recorded two assists in a 4–3 Lightning win over the Pittsburgh Penguins in Game 4 of the Eastern Conference Finals in the 2016 playoffs on goals by Ryan Callahan and Jonathan Drouin, respectively.

He surpassed former teammates Martin St. Louis and Vincent Lecavalier, who were tied for the most playoff games played in franchise history beforehand.

Hedman currently ranks third all-time in franchise history among defencemen in career points (229), behind Dan Boyle (253) and Pavel Kubina (243).

Hedman ranks third all-time among Lightning defencemen for games played, trailing only Pavel Kubina (662) and Cory Sarich (490).

[47] Hedman also joined Börje Salming, Nicklas Lidström and Erik Karlsson as the only Swedish defencemen to reach 70 points in a single season.

[51] On 21 April, Hedman was named as a finalist for the James Norris Memorial Trophy for the first time in his career, which is awarded annually to the NHL's top defenceman.

[52] Hedman finished third in Norris voting that season as the award ultimately went to Brent Burns of the San Jose Sharks.

[56] On 4 November, Hedman recorded two assists on goals by Steven Stamkos and Yanni Gourde in a 5–4 Lightning win over the visiting Columbus Blue Jackets.

[70] On 10 February, Hedman recorded his 400th career NHL point with an assist on a goal scored by Yanni Gourde in a 5–4 Lightning victory over the Pittsburgh Penguins.

[74] Despite the regular season dominance, the Lightning would surprisingly get swept in the first round of the 2019 playoffs by the eighth-seeded Columbus Blue Jackets.

Hedman was a finalist for the Norris Trophy for the third consecutive season, which eventually went to Calgary Flames captain Mark Giordano.

[75] On 5 December 2019, Hedman scored his 100th career NHL goal in a 5–4 loss against the Minnesota Wild against goaltender Alex Stalock.

[79] Hedman was named a finalist for the James Norris Memorial Trophy for the fourth consecutive season, which eventually went to Nashville Predators captain Roman Josi.

[80] On 23 August 2020, Hedman scored two goals in a 3–2 loss against the Presidents' Trophy-winning Boston Bruins on goaltender Jaroslav Halák in the first game of the second round of the 2020 playoffs.

On 16 March 2021, Hedman recorded his 500th point in a 4–3 overtime win against the Dallas Stars with an assist on a goal scored by captain Steven Stamkos.

On 17 April, Hedman recorded his 400th NHL assist on a goal scored by Erik Černák in a 5–3 loss to the Florida Panthers, becoming the first defenceman in franchise history to reach that mark.

Hedman was a Norris Trophy finalist for the fifth-straight season although the award ultimately went to Adam Fox of the New York Rangers.

After defeating the Toronto Maple Leafs, Presidents' Trophy-winning Florida Panthers and New York Rangers in the first three rounds of the 2022 playoffs, Hedman and the Lightning would clinch a third consecutive appearance in the Stanley Cup Finals (fourth altogether for Hedman's tenure and career) in which the Lightning would fall in six games to the top-seeded Colorado Avalanche.

[92] On 11 February 2023, Hedman recorded his 500th assist on a goal by Anthony Cirelli in a 3–1 victory over the Dallas Stars to become the 33rd defenceman in NHL history to hit the mark.

In the 2023 playoffs, Hedman and the Lightning would face the Toronto Maple Leafs in the first round for the second straight year, where this time they would lose in six games.

[99] On 16 January 2025, Hedman recorded his 600th career assist on a goal by Anthony Cirelli in a 4–3 SO win over the Anaheim Ducks, becoming the first player in Lightning history to reach the mark.

He later became one of the youngest players to play for Sweden's men's team when he made his debut at age 17 in an exhibition game against Norway.

Some notable players on the team included Peter Forsberg, Markus Näslund, Henrik Zetterberg and Nicklas Bäckström.