Early years As a youth, Mailloux played with the Toronto Marlboros of the
Greater Toronto Hockey League (GTHL) and participated in the
OHL Cup during his
midget year, earning selection to the tournament All-Star team.
Junior In the 2019–20 season, Mailloux played primarily for the
Ontario Hockey League (OHL)
London Knights'
Junior B affiliate, the
London Nationals, scoring 18 goals and 50 assists in a 48-game season that was prematurely shortened by the onset of the
COVID-19 pandemic in North America. For his efforts, he earned a number of
Greater Ontario Junior Hockey League (GOJHL) awards, including both rookie of the year and defensive player of the year honours. Mailloux also appeared in four games with the Knights during that year's
OHL season. 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. He then shared the images, and the identity of the woman, with members of his hockey team. Prior to the
2021 NHL entry draft, Mailloux renounced himself from the draft, stating that he did not want to be drafted, and wanted to focus on his reconciliation and growth. Despite this, the
Montreal Canadiens selected Mailloux with their first-round selection, 31st overall. 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.
Isabelle Charest, Quebec's Minister for the Status of Women, expressed that she was also surprised and disappointed by the pick. 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. On September 2, 2021, the OHL announced that Mailloux had been suspended due to the foregoing incident in Sweden. He was subsequently reinstated by the league on January 1, 2022. Mailloux played twelve games, managing three goals and six assists, before injuring his shoulder in a fight. The injury required surgery and brought his season to a premature end. Prior to the commencement of the
2022–23 OHL season, Mailloux was signed by the Canadiens to a three-year, entry-level contract on October 5, 2022. Team owner
Geoff Molson would later characterize him as "a good kid who made a big mistake and we’re going to help him along and we'll see how it all turns out." Appearing in 59 regular season games, he recorded 25 goals and 28 assists, the former output being the most for any defenceman that year. Mailloux earned praise for both his on-ice play and leadership qualities. He was subsequently named to the OHL Second All-Star Team. Qualifying for the ensuing playoffs, the Knights reached the OHL Finals, but were ultimately defeated by the
Peterborough Petes in six games. For his part, Mailloux recorded 24 points across 21 playoff games, setting a team record for a defenceman in a single postseason. Reflecting on being able to play a full season in the OHL for the first time, he said "I played some games here and have wanted to do that since I was 16. Getting that chance has been pretty special." He participated in the Canadiens' training camp in September 2023. On October 3, the Canadiens assigned him to the
Laval Rocket, the team's
American Hockey League (AHL) affiliate. Early assessments of his performance praised his offensive acumen, while suggesting that defensive responsibility would be an area for improvement to make the NHL. In January 2024, he was named the team's representative for the
2024 AHL All-Star Classic. 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. in September 2024 On April 15, 2024, Mailloux was recalled by the Canadiens prior to their final game of the
2023–24 NHL season. 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. 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. On October 7, 2024, Mailloux was again reassigned to the Rocket to begin the
2024–25 season. However, after fellow defenceman
Kaiden Guhle suffered an injury in a game on October 17, Mailloux was subsequently recalled by the Canadiens, and drew into the lineup on October 19, where he recorded his first NHL career goal in a 4–3 shootout loss to the
New York Islanders. After posting three points (1G, 2A) through five games, Mailloux was returned to the AHL ranks on October 30. In January 2025, Mailloux was named to the annual All-Star Classic for the second time in his career. Totaling 12 goals and 21 assists in 63 games for Laval, helping the team to the best regular-season record in the AHL and an appearance in the conference finals, he was selected as part of the league's Top Prospects Team.
St. Louis Blues On July 1, 2025, Mailloux's tenure with the Canadiens organization ended as he was traded to the
St. Louis Blues in exchange for
Zachary Bolduc. After a difficult start to the ensuing
2025–26 season in which he was held scoreless with a
plus/minus rating of -12 through nine games with St. Louis, Mailloux was reassigned to the team's AHL affiliate
Springfield Thunderbirds on November 9, 2025, prior to being recalled to the NHL ranks two weeks later. In January 2026, he agreed to a one-year extension with the Blues. ==Career statistics==