Going from an undrafted player to a Norris Trophy candidate, Girardi has transformed into a top-pair defenseman. An adept shot-blocker with acute defensive awareness, he is able to get himself out of trouble in his own end by moving the puck.

A native of Welland, Ontario, Girardi broke in as a 16-year-old with Barrie of the Ontario Hockey League in 2000-01 and helped it reach the OHL Finals in 2001-02. During the 2002-03 season, Girardi was traded to Guelph, where was an alternate captain. A year later he scored two goals and 19 points in the OHL Playoffs and helped Guelph win the championship.

Girardi was traded to London during 2004-05, and after combining for nine goals and 39 points during the regular season, helped lead London to the 2005 Memorial Cup championship.

Following his junior career, Girardi signed with Hartford of the American Hockey League but started 2005-06 with Charlotte of the East Coast Hockey League. After seven games, Girardi was promoted to Hartford, where he spent the remainder of the season, finishing second on the team among defensemen with 39 points. In 13 playoff games, Girardi had four goals and nine points.

The New York Rangers signed Girardi to an entry-level free agent contract on July 1, 2006, and assigned him to Hartford to start 2006-07. After 45 games Girardi was leading the team with 22 assists when he was called up. He made his NHL debut with the Rangers on Jan. 27, 2007, against the Philadelphia Flyers.

Girardi became a stalwart on defense, playing 330 consecutive games, a streak that ended on Jan. 24, 2011, because of a rib injury. Following 2011-12, Girardi finished sixth in voting for the Norris Trophy, awarded to the NHL's top defenseman, after scoring five goals and 29 points. He also had a plus-13 rating, 211 hits and 185 blocked shots.

Girardi has also become one of the premier shot-blockers in the NHL. On March 31, 2015, against the Winnipeg Jets, he became the first Ranger to have 10 blocked shots in a game since the NHL started keeping track of the statistic in 2005-06.

In 2014-15, Girardi had 30 or more minutes of ice time in three consecutive games (Nov. 1-5), becoming the first Rangers player to do so since Brian Leetch in 2000-01.

In Game 5 of the first round of the 2015 Stanley Cup Playoffs, Girardi appeared in his 94th postseason game with the Rangers, passing Walt Tkaczuk for most among skaters in franchise history. On Feb. 12, 2016, against the Los Angeles Kings, he became the 11th player to appear in 700 games for the Rangers.