[3] He spent his early years in Llandrindod Wells and moved to Cardiff at age 12,[4] where he attended Llanishen High School.

[9] His stage roles include Gorgio at the Bristol Old Vic, Assembly and A Good Night Out in the Valleys at National Theatre Wales, and The Sanger at Sherman Cymru.

[12] His television roles include Jonas in Black Mirror, drug trafficker Yoram in Banged Up Abroad[10][13] and Richard in Pen Talar.

[20] Cullen played Guy Fawkes alongside Kit Harington and Liv Tyler in Gunpowder on BBC One.

[22] Cullen made his directorial debut with the 2019 film Pink Wall starring Tatiana Maslany and Jay Duplass.

The film is described by Screen Daily as "the story of a couple’s struggles with the pressures of gender expectations and the conflict between life and ambition.