Warrant Officer Joel McMahon was born and raised in Perth, Western Australia and educated at Servite College. He enlisted into the Australian Army in 1998 and was allocated to the Royal Australian Infantry Corps as a rifleman.

On completion of initial employment training Warrant Officer McMahon was posted to the 3rd Battalion, The Royal Australian Regiment. During his tenure at the 3rd Battalion, Warrant Officer McMahon served at all levels from rifleman to Platoon Sergeant and Operations Sergeant.