Marchand grew up playing minor hockey in the Hammonds Plains, Nova Scotia area, including AAA midget with the Dartmouth Subways before being a second round pick in the 2004 QMJHL Midget Draft. He played four seasons in the QMJHL between the Moncton Wildcats, Val-d'Or Foreurs and Halifax Mooseheads.
Marchand was selected 71st overall in the 2006 NHL Entry Draft by the Boston Bruins. He made his NHL debut on October 21, 2009, against the Nashville Predators. He then scored his first career NHL goal against Jhonas Enroth of the Buffalo Sabres on November 3, 2010. Though a prolific goal scorer, Marchand is also known for his on the edge, and sometimes dangerous, play and has been suspended or fined by the league seven times.
Marchand's list of foul play includes a two-game suspension for elbowing R. J. Umberger in March 2011; a $2,500 fine for slew-footing Matt Niskanen in December 2011; a five-game ban for a predatory low-bridge hit on Sami Salo in January 2012; two games for slew-footing Derick Brassard in January 2015; and three games for clipping Mark Borowiecki in December 2015. In February 2017, the league docked Marchand $10,000, the maximum allowable amount per the collective bargaining agreement, for a dangerous trip of Detroit Red Wings defenceman Niklas Kronwall. In April 2017, Marchand was handed another two-game suspension for spearing Tampa Bay defenceman Jake Dotchin, marking the fifth time Marchand has been suspended for dirty and dangerous play.
Marchand established himself as a two-way player for the Bruins during the 2010–11 season, his first full year in the NHL. He scored 21 goals, including five short-handed (ranked third in the League), and 41 points; he also registered a +25 plus-minus rating. On April 2, 2011, prior to the Bruins' final home game of the season, against the Atlanta Thrashers, Marchand was awarded the Bruins' 7th Player Award, as voted by the club's fans through the regional New England Sports Network (NESN). The honour is awarded annually to the Bruins player fans believe most performed beyond expectations. During the 2011 Stanley Cup playoffs, Marchand scored 19 points over 25 games, helping the Bruins to a Stanley Cup championship. His 11 goals tied Jeremy Roenick for the second-most by a rookie in the NHL. Marchand's total included two goals in the seventh and deciding game of the Finals against the Vancouver Canucks.
During the off-season, the Bruins re-signed Marchand to a two-year contract extension, announced on September 14, 2011. During the ensuing season, on December 23, 2011, he scored his first career NHL hat-trick in an 8–0 win against the Florida Panthers at TD Garden. He also added two assists for a five-point effort. The following month, on January 9, 2012, Marchand was assessed a five-game suspension for illegally clipping Vancouver Canucks defenceman Sami Salo. The game was the teams' first meeting since the 2011 Stanley Cup Finals. Then-NHL disciplinarian Brendan Shanahan characterized the hit as "predatory" in his video release.
During the 2014–15 season, Marchand was suspended two games by the League for slew-footing New York Rangers forward Derick Brassard in the Bruins' 3–0 victory on January 15, 2015. Playing on the Bruins' top line for the majority of the season, he finished the season as Boston's leading goal-scorer, with 24 goals, and tied for fourth on the team with defenceman Dougie Hamilton in point totals, with 42. Immediately following the 2014–15 regular season, Marchand required surgery on the reportedly torn tendons around his right elbow, something that had been bothering him since the early spring of 2014 — he expected to be fully ready for the 2015-16 Boston Bruins season when training camp began on September 17, 2015.
On December 30, 2015 Marchand was suspended for three games for again illegally clipping an opposing skater, with the more recent infraction involving Marchand's play against Ottawa Senators defenceman Mark Borowiecki, resulting in Marchand being suspended from play in the 2016 NHL Winter Classic outdoor hockey game for the Bruins, and forfeiting nearly US $165,000 in fines from the illegal play.
During a home game on February 6, 2016 Marchand was given a penalty shot chance against the Buffalo Sabres, a rare occurrence during an overtime tie-breaker round in the regular season — with 2:28 left on the 3-on-3 overtime clock, Marchand scored on the penalty shot, the first time in Bruins team history that a successful penalty shot ever won a regular season game in overtime. Marchand finished the season with 37 goals and 24 assists, and a plus-minus of 21.
Midway through the 2016 World Cup of Hockey, where Marchand was one of the leading scorers with eventual champion Canada, he signed an eight-year contract extension with the Bruins worth $49 million through the 2024–25 season.
Late in the 2016–17 season, on March 13, 2017, Marchand scored the second hat trick in his Bruins career, with all three goals scored in the third period of a 6–3 road game Bruins win against the Vancouver Canucks – with only his teammate David Krejci scoring one additional goal in-between Marchand's trio of third-period Bruins goals.
Brad Marchand has represented Canada four times in the 2007, 2008 World Junior Ice Hockey Championships, 2016 IIHF World Championships, and 2016 World Cup of Hockey bringing home gold from each tournament. He scored the winning goal for Canada, short-handed, in the second and deciding game of the 2016 World Cup of Hockey final. He was the tournament's top goal-scorer and named to the all-star team.
As a result of his dirty playing style, fans commonly refer to Marchand as the "rat". During the Boston's 2011 playoff run, Marchand gained the nickname "Nose Face Killah,"a play on the stage name of the rap artist Ghostface Killah, as a result of his prominent nose. Among opposing fans, Marchand is often nicknamed "little ball of hate", with people mispronouncing his name on the Felger and Mazz show on 98.5 FM The Sports Hub in Boston sometime in October 2010. Among his Bruins teammates, Marchand is known as "Honey Badger." When the Bruins visited the White House after winning the 2011 Stanley Cup President Barack Obama described Marchand as their "Little Ball of Hate" in reference to his reputation as a pesty player.
Marchand is married to Katrina Sloane.