McTernan's career spans thirty years working in theatre, television and film. He has appeared with all the major theatre companies and began his career training and then performing at the Ensemble Theatre in Sydney. He has appeared in several productions there, most recently Wrong Turn at Lungfish in 2008. Previously at the Ensemble; Lovers, The Comedians, Boy Meets Girl, 6 Rms Riv Vu, Sonny and Same Difference.
