Monaghan was born in Berlin, Germany. At the age of twelve, his family moved from Germany to Heaton Moor in Stockport, Greater Manchester, and later to Manchester. Monaghan's first language is English, but he also speaks German and is well known for his ability to impersonate others and replicate accents.

He attended St. Anne's RC High School (where his uncle taught and later became head teacher) and Aquinas College, where he studied English literature, drama and geography.

Although he had many occupations when he was younger, he always wanted to be an actor. After his second year at Aquinas, he became a regular in school plays such as Oliver Twist, A Christmas Carol, and Bugsy Malone.

Monaghan then went on to join Manchester Youth Theatre where he was scouted for Hetty Wainthropp Investigates. He played Hetty's under-aged sidekick, Geoffrey Shawcross, for four seasons. He made his feature film debut as Russian sailor Sasha in TV film Hostile Waters.

Monaghan is an adamant nature lover, and has been active in the planting of trees, cacti, and the pod-seeds of mature hedge-row bushes. He owns a small forest in India, and has worked with the animal rights group PETA.[3]

Monaghan narrated Ringers: Lord of the Fans, a documentary about Lord of the Rings fandom. On his right arm, he has a tattoo of the Elvish word "nine" written in Quenya-mode Tengwar, a reference to his involvement in the Lord of the Rings and the fact that his character was one of the nine members of the Fellowship of the Ring.

Several other cast members also got similar tattoos during the film's production. On his left arm is a tattoo saying "Living is easy with eyes closed", a line from The Beatles' "Strawberry Fields Forever". He also has two stars tattooed on his left ankle.

Has been dating his former Lost co-star Evangeline Lilly since 2004. Last year the couple broke up, but in June 2008 Monaghan traveled to Argentina to film the pilot of the American version of 'CQC' (a successful television programme in more than five countries) and he was with Lilly. They were recently engaged.

Monaghan resides in Los Angeles, California. He is currently working on a script with LOTR co-star Billy Boyd.

He is a supporter of Manchester United Football Club.