Toronto Maple Leafs

Gardiner started the 2011–12 season with the Leafs, scoring his first career NHL goal on January 24, 2012, against Al Montoya of the New York Islanders.^[10] After his rookie campaign where he led rookie defensemen in scoring, Gardiner was named to the NHL All-Rookie Team.^[11]

Gardiner started his second season with the Toronto Marlies, due to the NHL Lockout. He posted 31 points in 43 games in the AHL, and then, once the NHL started up, he had 4 assists in 12 games. While playing with the Marlies, Gardiner suffered a concussion on December 8 and was out of the lineup for a month to recover.^[12] On May 6, 2013, Gardiner became the first Toronto Maple Leaf to score a home Stanley Cup playoff goal since Mats Sundin scored in a 3–2 loss to the Philadelphia Flyers on May 4, 2004, a span of nine years and two days.^{[citation needed]} He finished the playoffs with points in 6 games.

Gardiner scored a career high 10 goals and 31 points in the 2013–2014 season, trailing only Cody Franson in points among Leafs' defenders.^[13]

On July 29, 2014, the Leafs signed Gardiner to a five-year, $20.25 million contract extension.^[14] The Leafs struggled that season, and Gardiner managed only 4 goals and 24 points in 79 games, fourth among Leafs' defenders.^[13]

For the 2015–16 season, the Leafs hired a new head coach, Mike Babcock, and cleared out Dion Phaneuf by the trade deadline. Gardiner had 7 goals and 31 points, second to Morgan Rielly among Leafs' defenders, taking a step forward under a new coach, even though the team lost the scoring presence of Phil Kessel.

In 2016–17, the Leafs became a winning team once again, an especially high scoring one with the addition of scoring phenoms Auston Matthews, William Nylander, and Mitch Marner, among others. Gardiner posted a career high of 43 points, and was a +24, leading all Leafs' defenders in both categories.

During the 2017–18 season, Gardiner recorded his 50th point of the season on April 2, 2018, in a game against the Buffalo Sabres. With his 50th point, and fellow defenceman Morgan Rielly already having 51 points, they became the first two Leafs defensemen with at least 50 points in a season since Tomáš Kaberle and Bryan McCabe did it in 2006–07.^[15] Both Gardiner and Rielly ended the regular season with a career high 52 points to help the Leafs to their second consecutive playoff showing,^[16] in which they would lose to the Boston Bruins in seven games in the first round.

The following season, Gardiner played in his 500th career NHL game on October 27, 2018, against the Winnipeg Jets. In the game, he scored the tying goal in the third period to help the Leafs defeat the Jets 3–2.^[17]

Carolina Hurricanes

On September 6, 2019, Gardiner signed a four-year, $16.2 million contract with the Carolina Hurricanes.^[18]

Prior to entering his third season with the Hurricanes on September 7, 2021, it was announced that Gardiner was to undergo hip and back surgeries, which ruled him out for the entirety of the 2021–22 season.^[19] Gardiner was reported to be healthy prior to the 2022–23 season, but suffered a set back with his recovery and missed an entire second season.^[20][21] Gardiner's contract with the Hurricanes expired following the 2022-23 season.