[2] Heffernan was born in Billericay, Essex, England and worked as an usher at the National Theatre.
He has appeared on screen in a number of roles, including Henry Lascelles in the BBC adaptation of Susanna Clarke's novel Jonathan Strange & Mr Norrell,[3] Jaggers in Dickensian and Steven Rose in the fourth series of Luther.
In 2017, he played John Grigg, 2nd Baron Altrincham, in an episode of the Netflix series The Crown.
[4] He also played the Nine for Big Finish, in their Doctor Who box sets Doom Coalition and Ravenous.
[5] He returned to the National Theatre in 2022 to play the co-lead opposite Katherine Parkinson in a new production of Much Ado About Nothing directed by Simon Godwin.