[12] Harris picked up his first collegiate goal on October 13, 2018,[13] opening the scoring for Northeastern in their 5–0 shutout win over Sacred Heart.

[16] By the Huskies' holiday break, he had established himself as a playmaker for the team, with three goals and 13 points through 18 games, fifth among Hockey East defensemen.

[17] Harris won the 2020 Beanpot for Northeastern in double overtime, breaking their 4–4 tie against Boston University to give the Huskies their third title in a row.

[18] Harris's sophomore season came to a premature end due to the impacts of the COVID-19 pandemic, but the Huskies presented him with the Unsung Hero Award for his accomplishments.

[27] Harris declined to sign with the Canadiens after his junior year, fueling speculation that he would attempt to become an unrestricted free agent after graduating, but he told reporters that he was more invested in finishing his degree.

[29] He was named the Hockey East Defender of the Week on November 1 after scoring one goal and recording three assists in a two-game sweep of Maine.

[34] He debuted with the team on April 2, 2022, finishing the game with a plus–minus rating of +1 with three blocked shots in Montreal's 5–4 shootout win over the Tampa Bay Lightning.

[37] Harris represented the United States internationally at the 2020 World Junior Ice Hockey Championships in the Czech Republic.

[17] He scored his first goal of the tournament on the second day, taking a pass from Trevor Zegras to tie Team USA 1–1 against Germany; the US took the game in a 6–3 victory.