Fantilli during the 2024-25 season showed signs of living up to the promise that convinced the Columbus Blue Jackets to take him with the No. 3 pick in the 2023 NHL Draft. He scored three goals in a 7-3 win at the New York Rangers on March 5 to make him the first player with at least two hat tricks in a season at age 20 or younger since Patrik Laine had three with the Winnipeg Jets in 2018-19. It was part of a stretch when Fantilli had 10 goals in 16 games, making good on an opportunity for a bigger role with forward Sean Monahan sidelined with a wrist injury.

Fantilli signed his entry-level contact with the Blue Jackets on July 1, days after the 2023 draft. As a freshman at the University of Michigan in 2022-23, he led NCAA players with 65 points (30 goals, 35 assists) in 36 games and tied for the lead in goals. He won the Hobey Baker Award as the best men's NCAA ice hockey player and helped Michigan reach the Frozen Four semifinals. He also set a new Big Ten tournament scoring record with 11 points (seven goals, five assists) and was named most outstanding player and helped Canada win the gold medal at the 2023 IIHF World Junior Championship and the 2023 IIHF World Championship.

His list of accomplishments also include the Tim Taylor National Rookie of the Year award, first-team All-America, Big Ten Tournament MVP, Big Ten Freshman of the Year, College Hockey News and USCHO Player of the Year, first-team All-Big Ten, Big Ten All-Rookie Team and NCAA All-Regional Team honors.

Fantilli was chosen by Saginaw with the No. 18 pick in the 2020 Ontario Hockey League Draft, but already committed to play for Chicago in the United States Hockey League for the 2020-21 season. Competing as a 16-year-old for most of the regular season, Fantilli had 36 points (18-18-36) in 49 games, was named to the USHL All-Rookie Second Team after winning the Clark Cup and being named MVP of the playoffs with nine points (eight goals, one assist) in eight games, five points (four goals, one assist) in the Final that included two goals in the championship game.