Mikkola made his
Liiga debut playing with
KalPa during the
2014–15 Liiga season. On 29 May 2017, despite interest from the
St. Louis Blues to sign his entry-level deal, Mikkola opted to continue his professional career in the Liiga, signing a two-year deal with fellow Finnish outfit,
Tappara. By still harbouring NHL ambitions, Mikkola opted to attend the St. Louis Blues 2017 training camp in preparation for the
2017–18 season. He was assigned back to Tappara after competing in two pre-season contests with the Blues on 25 September 2017. In his only season with Tappara, Mikkola added 2 goals and 11 points in 50 regular season games before helping the club to a silver medal in the postseason, leading the league in plus-minus. On 1 June 2018, Mikkola agreed to a two-year, entry-level contract with the St. Louis Blues. During the
2019–20 season, on 7 January 2020, Mikkola made his NHL debut with the Blues against the
San Jose Sharks. On 8 March 2021, Mikkola scored his first NHL goal, against
San Jose Sharks goaltender
Devan Dubnyk. Following his second full season with the Blues in
2021–22, Mikkola as a restricted free agent was signed to a one-year, $1.9 million contract extension on 17 July 2022. During the
2022–23 season, Mikkola registered three assists through 50 regular season games before he was traded by the Blues on 9 February 2023, alongside
Vladimir Tarasenko, to the
New York Rangers in exchange for a conditional first-round selection in
2023 NHL entry draft and fourth-round selection in 2024,
Samuel Blais and Hunter Skinner. As a free agent from the Rangers, Mikkola signed a three-year, $7.5 million contract with the
Florida Panthers on 1 July 2023. On 24 June 2024, Mikkola won the
Stanley Cup with the Florida Panthers, after edging out the
Edmonton Oilers 2–1 in game 7 of the
2024 Stanley Cup Final. On 2 October 2025, Mikkola signed an eight-year contract extension with the Panthers beginning with 2026–27 NHL season. ==International play==