Born in Berlin, Germany to English-speaking parents, Big decided he wanted to be an actor during his second year of school when he was given a small role as one of Scrooge's workers in the play A Christmas Carol. He followed that up with roles in more school plays, including landing the plum role of the Artful Dodger in Oliver. Thoroughly bitten by the acting bug, he decided to look for more roles in plays outside of school. By the age of 12, his family had moved to Manchester, England, and he joined the Manchester Youth Theatre.

While studying English Literature, Drama and Geography at Sixth Form College, Monaghan worked at various jobs, including sorting mail for the post office, stocking shelves in a department store and working as a chef in a restaurant.

When he was only 18 years old, never having been on a film set before, Monaghan landed the lead role of Hetty Wainthropp's teenage sidekick, Geoffrey, in the television series Hetty Wainthropp Investigates, starring the famous British actress Patricia Routledge. He followed that up with a role in the TV movie Hostile Waters, starring Martin Sheen, filmed in England and Germany. Monaghan was filming a four-part mini-series called Monsignor Renard when he got the news that he?d landed the role of Merry in the Lord of the Rings trilogy. Naturally, he was thrilled to land a major role in a big budget Hollywood motion picture.

Naturally thin, Monaghan wore a fat suit to appear plump during filming. He has appeared on stage professionally in the British productions of The Resurrectionists as well as Annie and Fanny from Bolton to Rome. He also provided the voice of the male lead, Jason, in the BBC radio play Stockport.

Monaghan enjoys writing, soccer, scuba diving and surfing. He speaks German fluently and can imitate several different British accents. He currently lives in Los Angeles