Early career Sinisalo spent his youth career with
HyPS,
HJK and
Honka, and made his senior league debut for
FC Espoo in third-tier level
Kakkonen in June 2017, aged only 15.
Aston Villa He joined the
U21 academy at
Steve Bruce's
Aston Villa in 2018, and was a regular player in their
under-18 and under-23 sides. On 25 September 2020, Sinisalo joined
Ayr United in the
Scottish Championship on loan until the end of the season. He made his Ayr United debut on 6 October 2020, in a 5–2 away
Scottish League Cup victory over
Albion Rovers. On 8 April 2021, Sinisalo's loan was cut short, after he suffered a
groin injury and returned to Aston Villa. On 6 July 2021, Sinisalo was amongst several Academy players who were given new professional contracts. One year on, on 6 July 2022, Sinisalo renewed his Aston Villa contract once again. On 11 July 2022, Sinisalo signed for
League One club
Burton Albion on a season-long loan. He made his debut on 9 August 2022, in a 2–0
EFL Cup defeat to
Rochdale. The loan was ended early and Sinisalo returned to Villa on 1 January 2023. He made a total of eight appearances for the League One outfit. On 19 July 2023, Sinisalo joined
Exeter City on a season-long loan. He was given the number 1 shirt, and got a clean sheet on his debut, a 3–0 League One victory over
Wycombe Wanderers on 5 August 2023. During the season, Sinisalo made 50 appearances for the club in all competitions combined, recording 14 clean sheets. He was named The Player of the Year by the players and by the supporters of Exeter City, additionally winning four other awards in the club's end of season gala on 29 April 2024.
Celtic On 16 July 2024, Sinisalo signed for
Scottish Premiership club
Celtic on a five-year deal for an undisclosed fee, reported to be £1 million. He was given the number 12 shirt. On 8 February 2025, he made his debut for the club in a 5–0 home victory against
Raith Rovers in the
Scottish Cup. Following an injury to
Kasper Schmeichel whilst on international duty, Sinisalo made his league debut in a 3–0 home victory against
Hearts on 29 March 2025. After the game, his performance was praised by Celtic manager
Brendan Rodgers, who called him "excellent". In his second season, he again played second fiddle to Schmeichel for most of the season, only playing in a handful of cup games, though he was given the chance to start against
Stuttgart in the
UEFA Europa League on 26 March 2026, which was also his debut in
UEFA club competitions. He kept a clean sheet as Celtic won 1–0, though it was not enough to keep them in the tournament. Within the same week, due to Schmeichel suffering another injury, coinciding with a drop in form, Sinisalo played in an
Old Firm game which ended in a 2–2 draw at
Ibrox Stadium, and made an outstanding save against
Aberdeen to enable a 2–1 victory at
Pittodrie. ==International career==