With the OHL's pandemic hiatus halting the 2020–21 season, Mailloux was loaned to SK Lejon in the Swedish Hockeyettan, where he registered 15 points in 19 games.

While in Sweden, Mailloux received a 14,300 Swedish krona fine (approximately $1,640.00 USD or $2,000.00 CAD, at the time) for secretly taking photos of a young woman while they were engaged in a sexual act.

[2] The decision to draft Mailloux proved to be immensely controversial, attracting criticism from Canadian Prime Minister Justin Trudeau, a lifelong Canadiens fan, who announced that he was "deeply disappointed" by the team and characterized the selection as demonstrating a lack of judgment.

[5] On July 28, the Canadiens announced that Mailloux would not be invited to participate in the team's rookie or main training camp, and that they would "reassess" whether or not he was ready to join the organization later in the year.

[17] Early assessments of his performance praised his offensive acumen, while suggesting that defensive responsibility would be an area for improvement to make the NHL.

[19] Mailloux finished the season with 14 goals and 33 assists in 72 games played, and was named to the league's All-Rookie Team in recognition of his achievements.

[21] The following day, he made his NHL debut in a 5–4 shootout loss versus the Detroit Red Wings, registering an assist on a goal scored by Alex Newhook.

[22] After the Canadiens failed to qualify for the 2024 Stanley Cup playoffs, Mailloux was one of four players reassigned to Laval to end the season.