After several years with Arsenal Women's youth team, Flannery joined West Ham on 22 July 2022 at aged 16 feeling there was more opportunity for first-team football. West Ham were also the club she had supported all her life having once walked out with West Ham captain, Kevin Nolan as a mascot before a game at Upton Park.[1][2] She chose squad number 41, the same as her idol, Declan Rice.[3]
On 29 January 2023, she provided an assist in the 2–0 win over Wolverhampton Wanderers in the 2022–23 FA Cup,[5] impressing in the West Ham side and being named Player of the Match.[6]
On 9 November 2022, with England U18, she scored the only goal in the 5–1 defeat to Portugal.[15]
Flannery was named as part of the England U19 squad for Algarve Cup fixtures,[16] playing in matches against Portugal, Sweden and the Netherlands in November and December 2023.[17]