Norway Having started her youth career with her hometown team
Clausenengen, Mikalsen moved to
Arna-Bjørnar in August 2012. On 2 November 2013, Mikalsen made her senior debut, starting for Arna-Bjørnar in a 2–1
Toppserien defeat to
Vålerenga. In 2015, Mikalsen returned to
Clausenengen. She split playing time between the under-19s and senior
2. divisjon team. On 28 March 2016, Mikalsen made her debut for the team in a 1–1 draw with
LSK Kvinner.
Manchester United In July 2019, Mikalsen trained with
FA WSL club
Manchester United while the team was on tour in Oslo. On 6 September 2019, Manchester United made the permanent signing of Mikalsen, in need of a backup after injuries to
Emily Ramsey and
Fran Bentley coupled with the pregnancy of
Siobhan Chamberlain left the team with only
Mary Earps to start the season. She made her debut on 21 November 2019, starting in an 11–1
League Cup Group Stage victory over
Championship team
Leicester City. She left at the end of the season following the expiration of her contract having made one appearance.
Tottenham Hotspur On 27 July 2020, it was announced Mikalsen had signed a one-year contract with WSL team
Tottenham Hotspur. With Spurs in good form having won their last three matches and only conceded twice, Mikalsen was forced into a debut on 31 January 2021 with regular starter
Rebecca Spencer injured. She started in a 4–0 loss
Chelsesa. Mikalsen remained starter for the duration of Spencer's injury absence, playing a total of five games, all in the WSL. Spurs lost 1–0 to
Aston Villa, 3–2 to
Everton, and 2–0 to
Brighton & Hove Albion before Mikalsen earned her first and only clean sheet for Spurs as part of a goalless draw with
Reading. She left at the end of the season upon the expiry of her contract.
Sandviken On 28 July 2021, Mikalsen returned to Norway to sign with
Toppserien side
Sandviken.
1.FC Köln On 15 January 2025, it was announced that Mikalsen had signed for
Frauen-Bundesliga club
1. FC Köln until June 2026. ==International career==