[9] Marchand gained a reputation both for taking penalties and for riling his opponents during his minor hockey career, and he formed intense rivalries both with his Cole Harbour rivals and with his larger, stronger teammates.

[12] Shortly before the 2005–06 QMJHL season, Moncton hired Ted Nolan as their new head coach, and he took to Marchand's "drive and determination" at once, giving the skater an extensive role on the team.

[5] After the Mooseheads' loss in the QMJHL playoffs, the Bruins told Marchand that, rather than joining the team for the remainder of their season, he would be invited to summer training camp.

Marchand was frustrated with the decision, which was rumoured to be due to his on-ice attitude, and he entered the Bruins' 2008 training camp looking to rebuild his reputation and establish himself as a strong player.

[26] Originally, Marchand was meant to serve as the Bruins' spare fourth-line forward for the 2010–11 season, with Daniel Paille starting alongside Gregory Campbell and Shawn Thornton.

[28] By the end of the season, Marchand had 41 points (21 goals, 20 assists) in 77 contests, far fewer than fellow rookies Jeff Skinner, Logan Couture, and Michael Grabner, but he averaged significantly less time on the ice, and when he was asked to play, he was given few opportunities to score.

[29] Marchand also earned the first suspension of his NHL career during the season, receiving a two-game penalty on March 17, 2011, for elbowing R. J. Umberger of the Columbus Blue Jackets.

In the sixth game, Marchand was criticized by sportswriters for repeatedly punching Canucks star forward Daniel Sedin, an altercation for which he received no penalty.

[42] After battling their way to a Game 7 overtime, the defending Stanley Cup champion Bruins fell to the seventh-seeded Washington Capitals in the Eastern Conference quarterfinals.

[49] Going into the 2013–14 season with Tyler Seguin and Jaromír Jágr no longer on the Bruins, Marchand struggled to adjust to his new linemate Loui Eriksson, colliding with the other winger during a preseason game against the Winnipeg Jets on September 27, 2013.

Bruins head coach Claude Julien demoted Marchand from the second to the third and fourth lines on several occasions, giving players like Jordan Caron an opportunity to fill his position in the top-six.

[75] He nearly missed the game after meeting with the Department of Player Safety for a "dangerous trip" of Niklas Kronwall of the Detroit Red Wings, but ultimately received a $10,000 fine and no suspension.

[81] After having a career year in 2016–17, Marchand intended to double down on his consistent offensive production for the 2017–18 season with linemates Patrice Bergeron and David Pastrňák as they started being referred as the "Perfection" line.

[84] On January 24, 2018, Marchand received a five-game suspension from the NHL for an elbow to the head of New Jersey Devils forward Marcus Johansson the previous day, who suffered a concussion from the play.

[99] He and the Perfection line of him, Bergeron and Pastrňák continued to generate offensive power throughout the season, and on March 31, 2019, Marchand scored the 26th shorthanded goal of his career against Jimmy Howard of the Detroit Red Wings, breaking Rick Middleton's franchise record.

During the pause, Marchand was concerned that the NHL would attempt to resume operations too quickly, leading to "really, really ugly" games as unprepared skaters began playing without sufficient practice time.

[113] Despite Marchand scoring six goals in as many games during the 2020 playoffs,[114] the Bruins fell to the eventual Stanley Cup champion Tampa Bay Lightning in the second round.

[117] On September 14, 2020, Marchand underwent a sports hernia surgery that carried an expected four-month recovery time, leaving it in question whether he would be available for the start of the pandemic-shortened 2020–21 season.

[119] Marchand revealed that the sports hernia injury had been causing him pain for two and a half years, ended up on track to return to the Bruins at full strength for their January 14 home opener against the New Jersey Devils, in which he scored the team's first goal of the season on Devils' goaltender Mackenzie Blackwood and also recording an assist on a Nick Ritchie goal later on in the game as the Bruins won the game 3–2.

[120][121] On March 27, Marchand was placed on the NHL COVID protocol list, allowing young player Trent Frederic a chance to occupy his position on the top line with Bergeron and Pastrňák.

[127] Although he finished fifth in Hart Trophy voting, with Edmonton Oilers forward and captain Connor McDavid as a unanimous first selection and winner, Marchand was named to the 2021 All-NHL First Team at the end of June.

[137] He returned one month early of his initial plan, however, making his season debut on October 27, 2022, where he scored two goals and had an assist in a win against the Detroit Red Wings.

[140] With the retirements of both David Krejčí and longtime linemate and close friend Patrice Bergeron during the 2023 off-season, Marchand, along with Milan Lucic (who returned to Boston in 2023 in free agency), became the last active players from the Stanley Cup-winning 2010–11 Bruins roster still playing for the team.

In the first period of Game 3 of the Bruins second round matchup against the second-seeded and eventual Stanley Cup champion Florida Panthers, Marchand was the target of a sucker punch hit by Sam Bennett.

[159] Canada took first place in the tournament after defeating the previously undefeated Finnish team in a gold medal match that saw Marchand fight Leo Komarov of the Toronto Maple Leafs.

[157] His short-handed goal against Team Europe in the World Cup finals clinched a gold medal for Canada, something that Marchand and his teammates hoped would establish the skater as a strong player rather than a pest.

[165] In May 2019, teammate David Backes called the skater "the motor on [the Bruins]", while then-head coach Bruce Cassidy has noted that, although it may appear undisciplined, many of Marchand's tactics are calculated to get under the target's skin.

[168] When the Bruins visited the White House in January 2012 after their Stanley Cup victory seven months earlier, President of the United States Barack Obama referred to Marchand as a "Little Ball of Hate", a nickname that he has since embraced.

[169] His other nicknames include "Rat", inherited from previous Bruins pest Ken Linseman,[170] and "Leg Sweeper", for his frequent tactic of slew footing his opponents.

[172] In September 2016, just before the 2016–17 season began, he helped win the gold medal for Canada at the World Cup of Hockey, cementing his reputation as a strong skater and goal scorer in addition to an agitator.