Gardiner never played for the Anaheim Ducks after they selected him with the No. 17 pick of the 2008 NHL Draft; he was traded to the Toronto Maple Leafs, along with forward Joffrey Lupul, for defenseman Francois Beauchemin on Feb. 9, 2011.
He scored 30 points (seven goals, 23 assists) in 75 games in 2011-12, including his first NHL goal against the New York Islanders on Jan. 24, 2012. Gardiner was named to the NHL's All-Rookie Team and finished sixth in the voting for the Calder Trophy. He ranked first among rookie defensemen in assists and points.