His hometown National Hockey League (NHL) team was the Colorado Avalanche, and Carlo's favorite player was Rob Blake.

[12] In the second half of the season, he was paired predominantly with Mitch Topping and spent time on the Americans' penalty kill unit.

He did not significantly contribute offensively, providing only three goals and 10 assists in 71 regular season games,[13] but coach Jim Hiller praised Carlo's defensive abilities.

[18] He missed three games in February with an upper-body injury but returned to finish the season, helping the Americans to clinch a berth in the 2015 WHL playoffs with a goal during their regular-season finale against the Spokane Chiefs.

[19] Carlo completed the 2014–15 season with a career-high 25 points (four goals and 21 assists) in 63 games,[20] and the NHL Central Scouting Bureau ranked him No.

[25] Despite missing some time in the first half of the season with an injury, Carlo had two goals and 12 assists in 22 games by the Americans' mid-season break, and he had a strong defensive showing with partner Parker Wotherspoon.

[27] Despite Carlo scoring the game-winning goal in overtime to finish the regular season, the Americans fell short of clinching a playoff berth in 2016.

[3] When Chára was injured in November, Carlo was first paired with another veteran defenseman, John-Michael Liles,[34] and then with Kevan Miller, and he spent the captain's absence gaining confidence without the large, physical Chara controlling play.

[36] The Bruins clinched a berth in the 2017 Stanley Cup playoffs, but Carlo was prevented from making his postseason debut after suffering a concussion during Boston's regular-season finale.

[39] Entering the 2017–18 season, the Bruins paired Chára with another rookie defenseman, Charlie McAvoy, and Carlo was in turn partnered with Torey Krug.

[42] On March 31, 2018, Carlo suffered a season-ending ankle fracture while battling for a puck in the Bruins' game against the Florida Panthers, keeping him out of postseason contention for the second year in a row.

[59] By the time that the NHL suspended the 2019–20 season on March 12 due to the COVID-19 pandemic,[60] Carlo had four goals, 19 points, and a +16 rating, and the Bruins gave him the Eddie Shore Award for "exceptional hustle and determination" during their end-of-season banquet.

[61] He missed the last two games before the pause with a concussion after taking an elbow to the face from Evgenii Dadonov on March 5, and the season suspension allowed him time to recover.

[62] When the NHL resumed operations for the 2020 Stanley Cup playoffs in July, Carlo was one of 31 Bruins invited to play in the Toronto quarantine "bubble".

[65] With both Chara and Krug leaving the Bruins before the 2020–21 season, Carlo was paired with newcomer Matt Grzelcyk on the second defensive line to start the year.

Cassidy criticized the play as "a predatory hit" that targeted Carlo's head,[68] and the NHL suspended Wilson for seven games.

He appeared in an additional eight postseason games before the New York Islanders eliminated the Bruins in the second round of the 2021 Stanley Cup playoffs.

[72] On July 14, 2021, two weeks before he would have become a restricted free agent, Carlo signed a six-year, $24.6 million contract extension with the Bruins.

[74] Carlo first represented the United States in international competition when he skated with the under-18 team at the 2013 Ivan Hlinka Memorial Tournament in the Czech Republic and Slovakia.

[83] Carlo was raised in a non-denominational Christian household, and when he joined the Bruins, he began participating in a chapel group with several of his Boston teammates.