He played 204 games for Subiaco and Peel, captained Peel, and represented Western Australia at fullback.
He coached the Lions reserves to premierships in 2003 and 2005, and was an assistant coach under Peter German during the Lions' 2004 and 2006 premierships. When German stepped down in 2006, both German and most senior players favoured Monaghan to take over as senior coach, but the club board instead chose Scott Watters. In 2007 and 2008, Monaghan joined the Fremantle Football Club in a development role. He was appointed senior coach of West Perth for 2009, taking the team to the finals in his first year.
- Lewis, Ross (3 April 2009), "A bit of seasoning proved right recipe", The West Australian, Pre Game liftout, p. 10.