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, Stillman in order to continue his development joined the junior ranks of Danish club, Esbjerg Energy.
[3] 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.
Stillman turned professional full-time in 2023, joining the Utica Comets of the American Hockey League (AHL) for the 2023–24 season.