[1] Kastelic played his youth ice hockey years with the Phoenix Jr. Coyotes teams[2] while attending Desert Vista High School.

In 2021, Kastelic made his professional debut, joining the Belleville Senators of the American Hockey League (AHL), Ottawa's primary minor-league affiliate.

[7] On April 23, Kastelic scored his first two career NHL goals, including the eventual game-winner, in a 6–4 win against the Montreal Canadiens.

[12] On June 24, 2024, Kastelic was traded alongside Joonas Korpisalo and a 2024 first round pick to the Boston Bruins, in exchange for goaltender Linus Ullmark.

His father Ed is a former professional ice hockey player who played 220 games for the Washington Capitals and Hartford Whalers of the NHL.