A native of Truro, Nova Scotia, McIsaac played four seasons in the Quebec Major Junior Hockey League with the Halifax Mooseheads and Moncton Wildcats from 2016-20 prior to turning professional. In all, McIsaac compiled 160 points (33-127-160), a plus-89 rating and 248 penalty minutes in 205 QMJHL games, in addition to 20 points (2-18-20) and 48 penalty minutes in 36 postseason contests. In 2018-19, McIsaac was selected to the QMJHL Second All-Star Team after notching 62 points (16-46-62), a plus-33 rating and 56 penalty minutes in 53 games, along with 16 points (2-14-16) and 38 penalty minutes in 22 playoff matchups. McIsaac was also named to the QMJHL All-Rookie Team in 2016-17 and captured the Raymond Lagacé Trophy as the league's Defensive Rookie of the Year. On the international stage, McIsaac helped Team Canada win a gold medal at the 2020 IIHF World Junior Championship, tallying four points (1-3-4), a plus-four rating and eight penalty minutes in seven games. He also represented his country at the 2019 IIHF World Junior Championship, collecting one assist, a plus-four rating and four penalty minutes in four appearances. McIsaac competed at back-to-back IIHF World Under-18 Championships in 2017 and 2018, contributing five points (1-4-5), a plus-five rating and eight penalty minutes in 10 games. McIsaac previously earned a gold medal at the 2017 Ivan Hlinka Memorial Tournament, adding three points (1-2-3) and four penalty minutes in five games.
