Brigadier General Lenaghan joined the Australian Army in 1991, completing an Arts degree majoring in history and politics at the Australian Defence Force Academy.  He commissioned as an Infantry Officer in 1994.

His career has followed a path of platoon, company and battalion command within the 1st and 3rd Battalions of the Royal Australian Regiment. His broader service gave him the privilege of leading soldiers in dismounted, mechanised, airborne and amphibious roles. Outside of command, he filled operations and plans appointments at unit, formation, division and Army level.

He holds Bachelor and Master Degrees from the University of New South Wales and a Master in Business Administration from Deakin University.

His operational service includes deployments to Bosnia-Herzegovina, Afghanistan and Iraq.

Brigadier General Lenaghan was appointed the Deputy Force Commander for UNDOF as of 18th April 2025.