Girardi grew up playing minor hockey in his hometown of Welland, Ontario with the Welland Tigers of the OMHA’s Niagara District and South Central AAA Leagues. He was a member of the OMHA Minor Atom AAA Championship team in 1995 and an OMHA Finalist team in Minor Peewee in 1997.
Having gone undrafted in the 2003 draft, Girardi signed a free agent, two-way AHL contract with the Hartford Wolf Packof the AHL, the New York Rangers‘ AHL affiliate, and began the 2005-06 season with the Charlotte Checkers of theECHL. Eventually signed to an NHL deal by the Rangers while playing with the Wolf Pack, Girardi was called up in January 27, 2007, after an injury to Darius Kasparaitis. Since that callup, Girardi has dressed for every regular-season and postseason game that the Rangers have played, except for two games in the 2010–11 season when Girardi was sidelined with a rib injury in late January 2011 (which ended a consecutive games-played streak at 330).
On February 16, 2008, Girardi signed a two-year, $3.1 million contract extension with the Rangers.
On July 9, 2010, Girardi re-signed with the Rangers, signing a four-year, $13.3 million deal. During the 2010-2011 NHL season, Girardi led the NHL in blocked shots with 236.
For the 2011–12 season, Girardi was named an interim alternate captain for the Rangers while Marc Staal was recovering from post-concussion syndrome. In Staal’s absence, Girardi led all NHL skaters in average time on the ice, averaging nearly 28 minutes a game. His play earned him a spot in the 2012 NHL All-Star Game. Girardi scored his first playoff goal on April 26th, 2012 versus the Ottawa Senators during game 7 of the 2012 Eastern Conference Quarter Finals.