[3] Prior to his senior year at Minnetonka High School, Gardiner switched from forward to defense on the advice of Minnesota State University, Mankato coach Troy Jutting.

[4] While with the Badgers, on February 9, 2011, Anaheim traded Gardiner, along with Joffrey Lupul and a conditional fourth-round draft pick in 2013, to the Toronto Maple Leafs in exchange for defenseman François Beauchemin.

[7] Gardiner chose to forgo his senior season with the Badgers and instead signed a three-year entry-level contract with the Toronto Maple Leafs.

[8][9] 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.

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.

[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.

After the Toronto Maple Leafs failed to make a playoff run in the 2013–14 season, Gardiner joined teammates Nazem Kadri, James Reimer and Morgan Rielly to compete in the 2014 IIHF World Championship.

[23] On July 9, 2017, Gardiner married his long-time girlfriend Lucy, a fitness instructor, businesswoman, and social media influencer.