Gardiner was a latecomer to pro hockey. He was 29 when he joined the Calgary Tigers of the Big 4 Hockey League in 1920-21; one season later, the Tigers became part of the new Western Canada Hockey League.

The Winnipeg native quickly became one of the WCHL's top defensemen, forming an elite duo with Red Dutton. He was a key to Calgary's run to the WCHL championship in 1924, and though the Tigers lost to the Montreal Canadiens in the Stanley Cup Final, Gardiner left an impression.

When the WCHL went out of business after the 1925-26 season, the Canadiens bought his rights and brought him to Montreal. Despite being a 35-year-old playing on a rebuilding team, Gardiner combined with Sylvio Mantha to form one of the NHL's best defense pairings. He often played all 60 minutes in a game, and was voted winner of the Hart Trophy as League MVP in 1926-27.

Gardiner played for the Canadiens in 1927-28, then was loaned to the Chicago Black Hawks, where he served as a player-coach for most of the 1928-29 season before being recalled by the Canadiens for the Stanley Cup Playoffs. He finished his NHL career with 20 points (10 goals, 10 assists) in 100 games.

After leaving the NHL, Gardiner became a player-coach with Philadelphia of the Canadian-American Hockey League (later the American Hockey League). He remained a coach with AHL and Eastern League teams in Philadelphia until 1946.

Gardiner was inducted into the Hockey Hall of Fame in 1958. When the Philadelphia Flyers were awarded an NHL franchise and entered the League in 1967, owner Ed Snider discovered Gardiner was retired and living in Philadelphia, and he made him the new team's first season ticket holder. Gardiner remained an avid fan of the Flyers until his death on Jan. 11, 1972.