She attended Holy Child Killiney before going on to train at the Gaiety School of Acting, graduating in 2014.
[4] Upon graduating from the Gaiety, Mullen made her television debut in an episode of the Damo and Ivor comedy show before being cast as Katie Kiely in the Virgin Media One crime drama series Red Rock, a main character Mullen played for the first two series from 2015 to 2017.
[7] In 2020, Mullen made her London stage debut in Meat at Theatre503 and played Peggy, Marianne's (Daisy Edgar-Jones) friend from Trinity College Dublin, in the BBC Three and Hulu miniseries adaptation of Sally Rooney's Normal People.
[10] She had a recurring role as Mary Pickett in the 2022 Amazon Prime science fiction series The Peripheral.
Mullen is set to star opposite Allen Leech in the true crime drama The Vanishing Triangle.