Ylönen played as a youth with his hometown club, the Espoo Blues, through to the under-18 level. After a stint within the Jokerit junior program, Ylönen returned to the Blues making his professional debut with Espoo United of the Mestis during the 2016–17 season. In his first full season with Espoo United in 2017–18, Ylönen posted an impressive 27 points in 48 games as a rookie.
In his final year under contract with the Pelicans in the 2019–20 season, Ylönen was unable to surpass his rookie season production, registering 12 goals and 22 points through 53 games. On 1 March 2020, with the Pelicans out of playoff contention, Ylönen opted to sign his first professional contract abroad, agreeing to play the remainder of the season with the Canadiens' American Hockey League (AHL) affiliate, the Laval Rocket.[5] However, due to the COVID-19 pandemic, his debut with the team would be postponed.[6] On 26 March, Ylönen was signed to a three-year entry-level contract with the Canadiens.[7] With the pandemic delaying the beginning of the 2020–21 NHL season, Ylönen was loaned back to the Pelicans to start the 2020–21 campaign, subsequently returning to Montreal with the opening of their respective training camp.[8] Ylönen would eventually make his North American debut during the 2020–21 AHL season.[7] After scoring nine goals and 17 points with the Rocket, Ylönen would be recalled by Montreal on 12 May 2021.[9] He made his NHL debut in a 4–3 overtime loss to the Edmonton Oilers that same day, being the final game of the season.[10]
During the 2022–23 season, Ylönen was recalled by the Canadiens on 11 January 2023 after posting eight goals and 27 points in 34 games with Laval.[16] He scored his first goal of the season and likewise had his first multi-point game in the NHL during a 5–2 win over the Philadelphia Flyers on 24 February 2023.[17] Ylönen was then assigned to Laval on 9 April having played in 37 games with Montreal, scoring six goals and 16 points.[18]
On 31 July 2023, Ylönen signed a one-year two-way contract extension with the Canadiens.[19]