In 2003, at the age of 15, Nathan McMullen was named "Young Comic of the Year" by Liverpool Echo during the Liverpool Comic Festival. More than 300 teenagers aged 14 to 18 competed in the contest for 12 months. For his win, Nathan got 250 pounds. He studied acting at the Manchester School of Theatre.
His first professional roles came in 2010, when he starred in the TV series Casualty and played Jack in Jack and the Beanstalk at the West Yorkshire Playhouse in Leeds. In 2011, he started working for the Everyman Theatre, where he starred as Witch 3 and Feance in Macbeth.
In 2012, McMullen appeared in the fourth season of Misfits in the regular role of Finn. McMullen plays the part of Wolf the elf in an episode of Doctor Who.
In 2015, McMullen participated in the stage adaptation of the 1994 comedy film The Hudsucker Proxy by the Coen brothers. Premier of the play was delayed because McMullen was injured during rehearsals. In 2016, McMullen was a member of the cast of the classic play by Alan Bleasdale Down the Dock Road.
Nathan McMullen will continue his work for Everyman Theatre in 2018. This season he will star in the plays Paint Your Wagon (as Mike Mooney), The Big I Am (based on Peer Gynt) and A Clockwork Orange. He also works as a tutor in an acting school Act Up North, which has divisions in Manchester, Liverpool and Leeds.