Franssi has played in the youth sectors of ,
Hyvinkään Palloseura, Koivukylan Palloseura and .
First spell at PK-35 Franssi made her league debut against
Åland United on 15 April 2012. She scored her first league goal against
GBK on 23 March 2016, scoring in the 9th minute. In 2016 Franssi was named
Naisten Liiga Player of the Season after hitting 20 league goals for her club, PK-35.
FC Zürich Frauen Franssi spent the 2016–17 season with
FC Zürich Frauen, scoring 16 goals and finishing as runner-up to
FC Neunkirch in both the
Nationalliga A and
Swiss Women's Cup.
Second spell at PK-35 Franssi returned to PK-35 in July 2017. She made her league debut against
JyPK on 8 July 2017. Franssi scored her first league goal against
TPS on 15 July 2017, scoring in the 32nd minute.
Juventus Franssi signed for newly founded
Serie A club
Juventus in September 2017. She made her league debut against
Atalanta on 30 September 2017. Franssi scored her first league goal against
Empoli on 28 October 2017, scoring in the 10th minute.
Fortuna Hjørring Franssi made her league debut against
Odense Boldklub Q on 8 September 2018. She scored her first league goal against
B 93 on 16 September 2018, scoring in the 67th minute. On 27 April 2019, Franssi extended her contract by one year.
Real Sociedad On 17 July 2020, Franssi was announced at Real Sociedad. She made her league debut against
Deportivo de La Coruña on 11 October 2020. Franssi scored her first league goal against
Dux Logroño on 11 November 2020, scoring in the 70th and 79th minute. In December 2021 Franssi was playing for
Real Sociedad of Spain's
Primera División but was reported to be attracting
transfer interest from multiple clubs in England's
FA Women's Super League. On 6 April 2022, Franssi's contract was extended for two years.
London City Lionesses On 4 July 2025, newly promoted
Women's Super League club
London City Lionesses announced the signing of Franssi. ==International career==