Monaghan had been spotted in a play in London and was asked to audition for a role in "The Lord of the Rings". While it took the producers several months to decide, he landed the plum role of the Hobbit named Merry. Shot over two years in 1999 and 2000, the film adaptation was a large gamble for New Line. The studio released the project in three parts over three years, "Lord of the Rings: The Fellowship of the Ring" (2001), "Lord of the Rings: The Two Towers" (2002) and "Lord of the Rings: The Return of the King" (2003).
Rather than find himself typecast in subsequent fantasy or period fare, Monaghan headed to Hollywood in search of different roles and was subsequently cast as Charlie, the wanting-to-be-recognized rock drummer among the 48 marooned airline crash survivors on J.J. Abrams' clever adventure drama "Lost" (ABC, 2004-).