Although Monaghan held many occupations when he was younger, he always wanted to be an actor. After his second year at St. Anne's, Monaghan became a regular in school plays such as "Oliver Twist", "A Christmas Carol" and "Bugsy Malone" before joining the Manchester Youth Theatre. There, he was scouted for "Hetty Wainthropp Investigates", in which he starred alongside Patricia Routledge and Derek Benfield as Hetty's under-aged sidekick, Geoffrey Shawcross, for four series. Monaghan has since credited Routledge as "an amazing teacher" who taught him some "very valuable lessons" that have influenced his acting career. He made his feature-film debut as Russian sailor Sasha in the TV film "Hostile Waters".