The boss of the Defence Department, Dennis Richardson, will step down next month after a five decade public service career.

He will leave his Canberra office on May 12, two days before his 70th birthday.

Dennis Richardson has spent 48 years in public service roles and national security. Credit:Andrew Meares

Respected former Communications Department chief Drew Clarke has also drawn the curtain on a distinguished public service career with his resignation as Chief of Staff to Prime Minister Malcolm Turnbull.

Mr Richardson has led the mammoth department since 2012. His tenure at Defence was part of a 48-year contribution to public service and national security, Mr Turnbull said on Wednesday morning.