[5] During the 2015–16 NCAA men's ice hockey season, BU coach David Quinn paired McAvoy with star BU defenseman and future fellow NHL player Matt Grzelcyk, already selected as a third round 2012 NHL Entry Draft pick by the Boston Bruins.

[8] Leading up to the 2016 NHL Entry Draft, McAvoy was highly regarded by scouts and was considered one of the top four defensemen available.

[14] On December 18, 2017, McAvoy scored a Gordie Howe hat trick, which is an unofficial stat where a player gets a goal, an assist, and a fight in one game, against the Columbus Blue Jackets.

[15] Shortly after New Years' 2018, McAvoy was operated on for symptoms of what was diagnosed as supraventricular tachycardia, evidenced by an episode of heart arrhythmia following the Bruins' November 26, 2017 home game against the Edmonton Oilers.

[17] By February 1, McAvoy had resumed full participation with his teammates in Bruins team practice sessions at the Warrior facility.

[22] Following the Bruins defeat in the second round of the 2018 Stanley Cup playoffs, McAvoy was named to the senior United States roster to compete at the 2018 IIHF World Championship.

[30] McAvoy's sophomore season would end in heartbreak as the Bruins would fall to the St. Louis Blues in Game 7 of the 2019 Stanley Cup Finals.

[31] McAvoy was expected to take a big step forward in his production starting in the 2019–20 season, playing alongside NHL legend and Bruins captain Zdeno Chára.

In the 2020 Stanley Cup playoffs, playing in a bubble due to the pandemic, McAvoy recorded one goal and three assists, as the Bruins fell in five games in the second round to the Lightning.

Although his offensive numbers were seen as low, he took a huge step defensively, and finished 10th in Norris Trophy voting, awarded to the league's best defensemen.

In the 2020 offseason, both Chara and Torey Krug, important parts of the Bruins defense corps, left in the free agency.

Once again set to be a restricted free agent, on October 15, 2021, McAvoy signed an eight-year, $76 million contract extension with the Bruins.

On June 3, 2022, the Bruins announced that McAvoy underwent shoulder surgery and would miss six months, sidelining him for the beginning of the 2022–23 season.

After that, it was smooth sailing for McAvoy and the Bruins, who would set the NHL single-season records with 65 wins and 135 points.

On October 30, 2023, McAvoy delivered an illegal check to the head on Florida Panthers defenseman Oliver Ekman-Larsson, who was injured on the play.

[44] McAvoy married his longtime girlfriend in August 2023,[45] who is the daughter of the Pittsburgh Penguins' head coach Mike Sullivan.