Stillman played major midget hockey within the Sudbury region before he was selected by the
Sudbury Wolves, in the second round, 25th overall, of the 2019
OHL Priority Selection. In his first season of major junior hockey with the Wolves in
2019–20, Stillman showed offensive potential in recording 13 goals and 34 points through 58 games and was subsequently named to the OHL First All-Rookie Team. With the
COVID-19 pandemic suspending and later cancelling the
2020–21 season, to continue his development, Stillman joined the junior ranks of Danish club
Esbjerg Energy. Producing 16 points through only 8 games at the under-20 level, Stillman was later drafted by
New Jersey Devils in the first round of the
2021 NHL Entry Draft with the 29th overall selection. He was later signed to a three-year, entry-level contract with the Devils on August 21, 2021. Returning to the OHL for the commencement of the
2021–22 season, Stillman, as an alternate captain, notched 20 points through 24 appearances. On January 10, 2022, Stillman was traded by the Wolves to the
Peterborough Petes in exchange for Alex Pharand and five draft picks. Through the second half of the campaign, Stillman contributed with 10 goals and 29 points in 35 regular season games. After a first-round playoff defeat, Stillman was reassigned by the Devils to join their
American Hockey League (AHL) affiliate, the
Utica Comets for their playoff run on May 1, 2022. He ultimately did not play for Utica that year. Stillman returned to the Petes for the
2022–23 season, and scored 48 points in 59 games, a similar per-game rate to his previous season. Stillman turned professional full-time in 2023, joining Utica for the
2023–24 season. He scored his first professional goal on October 28, 2023, in a game against the
Rochester Americans, for whom his brother was playing. Stillman would go on to add a second goal in the 5–4 Comets loss. He scored his first professional
hat trick on February 25, 2024, in a 5–2 victory over the
Syracuse Crunch. He finished his first AHL season with 14 goals and 24 points in 54 games. During the season, Stillman was recalled by the Devils from the Comets, however he remained a healthy scratch through his tenure with the Devils. At the NHL trade deadline on March 7, 2025, Stillman was dealt by the Devils alongside the rights to prospect Max Graham and a third-round pick in 2027 to the
Pittsburgh Penguins in exchange for
Cody Glass and
Jonathan Gruden. On July 13, 2025, Stillman, along with a 4th-round pick in the 2027 NHL entry draft, was traded by the Penguins to the Vancouver Canucks in exchange for 2025 Calder Cup MVP goaltender
Artūrs Šilovs. ==Personal life==