Cooper was previously the Liberal Democrat spokesperson for Health, Wellbeing and Social Care from October 2021 to September 2024, spokesperson for Education from September 2020 to October 2021, and the spokesperson for Justice and for Digital, Culture, Media and Sport from January 2020 to September 2020.
Cooper stood in the 2015 Lewes District Council election held on the same day; she was elected to represent the Lewes Bridge ward. Cooper stepped down as a councillor in 2016.[12]
Cooper was elected to Parliament as MP for St Albans at the 2019 general election with 50.1% of the vote and a majority of 6,293.[16][17][18]The Guardian named Cooper as one of the ten new MPs from all political parties to "watch out for".[9]
In January 2020, it was announced Cooper had been appointed as the Liberal Democrats' justice, culture, media and sport spokesperson.[19] In June, she took part in George Floyd protests in Verulamium Park, St Albans, where she gave a speech about police brutality.[20]
In September 2020, Cooper was announced as the party's new deputy leader and education spokesperson.[21]
In May 2021, Cooper was a signatory to an open letter from Stylist magazine, alongside celebrities and other public figures, which called on the government to address what it described as an "epidemic of male violence" by funding an "ongoing, high-profile, expert-informed awareness campaign on men’s violence against women and girls".[22]
Cooper was re-elected as MP for St Albans at the 2024 general election with an increased vote share of 56.6% and an increased majority of 19,834.[23] During the election period, Cooper had participated in the televised leaders debates.[24]