She began her career as a child actress, making her debut in Marina Carr's By the Bog of Cats (1998) at the Abbey Theatre.

[5] Cullen was about eight when she was cast in her first professional play as Josie Kilbride in Marina Carr's By the Bog of Cats at the Abbey Theatre.

Also in 2012, she starred in Eoin Macken's horror film The Inside and made her New York stage debut in The Life and Sort of Death of Eric Argyle at 59E59 Theater.

In 2016, Cullen appeared in the films The Randomer and The Limehouse Golem, and made her London stage debut with a small role in The Plough and the Stars at the National Theatre in 2016.

She starred opposite Gemma-Leah Devereux in the film The Bright Side, had a main role in the ITV crime drama The Long Call as Caroline Reasley, and made a guest appearance in an episode of Dalgliesh on Channel 5.