McQuaid played major midget hockey in his hometown of Cornwall, Prince Edward Island, for the Cornwall Thunder before playing major junior hockey with the Sudbury Wolves of the Ontario Hockey League for four seasons. McQuaid was selected in the second round, 43rd overall, in the 2003 OHL Priority Selection as a 17-year-old overage player. He was undrafted by the OHL the previous season. After scoring 19 points in his second season with the Wolves in 2004–05, McQuaid returned to the OHL and, in his final season, helped lead the Wolves to the OHL Final against the Plymouth Whalers; the Wolves were defeated in six games
Professional
McQuaid was selected in the second round, 55th overall, by the Columbus Blue Jackets in the 2005 NHL Entry Draft. He returned to the OHL and did not play a game with Columbus before he was traded to the Boston Bruins in exchange for a fifth-round draft pick in the off-season. He was immediately signed by Boston to a three-year, entry-level contract. He was assigned to Boston's American Hockey League affiliate, the Providence Bruins in 2007–08 and scored nine points in his professional rookie season. He scored his first NHL goal on February 7, 2010, a game-winner against goaltender Jaroslav Halák in a 3–0 win against the Montreal Canadiens at the Bell Centre. During Game 4 of the 2011 Stanley Cup Finals, analyst Pierre McGuire described him as "one tough hombre." In that same final, McQuaid won his first Stanley Cup. On July 14, 2011, McQuaid signed a three-year, $4.7 million contract with Boston. In 2010, McQuaid suffered a freak concussion after tripping over his suitcase. In the 2012–13 season, McQuaid helped the Bruins to the Stanley Cup Final by scoring the game-winning goal against the Pittsburgh Penguins in Game 4 of the Eastern Conference Final. He and the Bruins would end up losing in the Final to the Chicago Blackhawks in six games. In the 2013–14 season, McQuaid would miss most of the season with an illness. On June 26, 2015, McQuaid signed a four-year, $11 million contract extension with the Bruins. In a game against the Washington Capitals on January 5, 2016, Capitals forward Zach Sill boarded McQuaid and was suspended two games. On October 19, 2017, McQuaid was placed on injured reserve after breaking his right fibula in a game against the Vancouver Canucks. Having played in nine straight seasons with the Bruins and approaching the 2018–19 season, McQuaid was traded to the New York Rangers in exchange for Steven Kampfer, a 2019 fourth-round pick and a conditional seventh-round pick on September 11, 2018. McQuaid added a physical presence on the blueline of the rebuilding Rangers, and recorded 2 goals and 5 points through 36 games. With the Rangers out of playoff contention, and in his final year under contract, McQuaid was traded by the Rangers at the trade deadline to his original draft club, the Columbus Blue Jackets, in exchange for Julius Bergman, and a fourth and seventh round picks in 2019 on February 25, 2019. On March 15, 2019 McQuaid scored his first goal as a Blue Jacket, the game-winning goal as they shut out the Carolina Hurricanes, 3–0 in Columbus.
