[3] He was considered the top Canadian prospect in the draft, however, he had already committed to play for the Chicago Steel.

[4] On September 13, 2021, the Spirit exercised their defected player rule and traded Fantilli's OHL playing rights to the North Bay Battalion.

On March 25, 2020, Fantilli signed a USHL tender agreement with the Chicago Steel for the 2020–21 season.

[7] During the regular season he recorded 18 goals and 18 assists in 49 games for the Steel and was named to the USHL All-Rookie Second Team.

[14] He made his debut for Michigan on October 7, 2022, in a game against Lindenwood, where he won 15 of 19 faceoffs and recorded two assists.

[17][18] The next day, he scored a career-high four points with one goal and three assists, in a victory over Lake Superior State.

[29][30] He became the third freshman to win the Hobey Baker Award, after Paul Kariya in 1993 and Jack Eichel in 2015.

[34] He scored his first career NHL goal on October 21, 2023, as part of a 5–4 overtime win against the Minnesota Wild.

[40] Fantilli was part of the Canadian boys' team at the 2020 Winter Youth Olympics where he served as alternate captain.

[42][43] In 2022, Fantilli was named to Canada's national under-18 team at the 2022 IIHF World U18 Championships, where he again served as alternate captain.

[47] On May 5, 2023, Fantilli was named to Canada's senior national team at the 2023 IIHF World Championship.

[52][53] After missing the final months of the 2023–24 season with injury, Fantilli rejoined the senior national team for the 2024 IIHF World Championship.

"[37] However, Fantilli was subsequently dropped from the roster in advance of the tournament, with the addition of a number of more experienced NHL players whose teams were ousted in the first round of the playoffs.