Cassidy was born in Salford. She is the older sister of actress Raffey Cassidy, who portrayed Athena in Tomorrowland. Cassidy joined the National Youth Theatre. She graduated with a Bachelor of Arts in Drama and Theatre Arts from Goldsmiths, University of London in 2019. She then trained at the Royal Academy of Dramatic Art (RADA), graduating in 2021 with a Master of Arts in the Theatre Lab programme.
