Monaghan worked as a teacher prior to entering politics and has lived in the Northern Territory for over 20 years.
After teaching, Monaghan was attached to various roles, including being the General Manager of Engineers Australia.
[3] Monaghan was selected for Labor to contest the marginal seat of Fong Lim in the 2020 Northern Territory general election to replace Jeff Collins who resigned from the party in 2019 and contested the seat with the Territory Alliance party.
Monaghan won this seat against incumbent, Jeff Collins, despite it being a marginal victory, it was one of the competitive races which decided the 2020 election.
Monaghan was selected by his caucus as speaker in 2023 after a struggle to fill the speakership following the resignation of Kezia Purick.