Sullivan has been a Councillor on Gravesham Borough Council since 2015. She has been a Cabinet Member since 2019 and resigned as a Cabinet Member following her election to Parliament.
Sullivan has been a Councillor on Kent County Council since 2017. Since 2021, she has been the Leader of the Opposition at Kent County Council, a position from which she resigned following her election to Parliament.
Sullivan has been criticised for her vote to cut the winter fuel allowance for pensioners. Sullivan has also been criticised for keeping her roles as a Councillor at Gravesham and Kent councils, securing the nickname three jobs Sullivan from constituents. It is believed that Sullivan will stand down from both positions for the 2025 local elections. Sullivan also received criticism for a Twitter post detailing the location of child refugees during the 2024 UK riots.