As reported on Defence Connect, Prime Minister Malcolm Turnbull confirmed Richardson's departure yesterday.
The Prime Minister's statement said Richardson's resignation will take effect from 12 May 2017.
"In this near half century of service, Mr Richardson has served 12 prime ministers with consistent professionalism. His commitment to public service, his strategic insight, his candour and integrity have made him a trusted adviser to governments from both sides of Australian politics," said Prime Minister Turnbull.
"On behalf of the government and the Australian people, I thank Mr Richardson for his leadership and service and I wish him and his wife, Betty, all the very best for the future."
The Prime Minister touched on Richardson's distinguished career in the public service over the last 48 years.
"Mr Richardson has had a long and distinguished career and has made a significant contribution to Australia’s national security and foreign policy over 48 years of public service. He was appointed as secretary of Defence in 2012 having served previously as the secretary of the Department of Foreign Affairs and Trade (2010-2012), Australian ambassador to the United States of America (2005-10) and the director-general of security (1996-2005). He previously worked in the departments of Immigration and Prime Minister and Cabinet, and served as principal adviser to prime minister Hawke from 1990 to 1991."
Minister for Defence Senator Marise Payne paid tribute to Richardson and his service to national security and defence.
"Dennis has had a singular 48-year career in the public service. His record of service in national security, foreign affairs and defence is incomparable and speaks for itself," said Minister Payne.
"Since I have become minister, I have never been left in any doubt about Dennis' views. His judgement, insight and unparalleled experience ensured he has been an invaluable source of counsel to me. His depth of knowledge, experience and his plain speaking will be missed."
Minister for Defence Industry Christopher Pyne also spoke highly of Richardson and his professionalism and dedication to Australia.
"Mr Dennis Richardson AO has dedicated his life to promoting Australia’s interests both at home and abroad. He has been a trusted adviser to Australian governments for almost five decades," he said.
"Dennis’ professionalism and integrity are the very embodiment of the spirit that has driven the Australian public service for over a century.
"Dennis’ frankness and fearlessness in counsel is legendary, I can personally attest to that. His contribution to the country as secretary of the Department of Foreign Affairs and Trade, ambassador to the United States, the director-general of security and finally in his role as secretary of the Department of Defence, will leave a lasting legacy.
"Dennis’ role in implementing the First Principles Review will mean he will leave a Department of Defence that is ready to face the new challenges presented to it in the coming decades. He has also guided through the Defence White Paper that will transform our Defence Forces, and build an Australian defence industry capability, a great new national endeavour for our country."
Chief executive of Australian Industry (Ai) Group Innes Willox also paid tribute to the former diplomat.
"The decision by Dennis Richardson to resign from Defence will see the departure of one of Australia's most dedicated, long serving departmental heads," said Willox.
"On behalf of Ai Group and the Ai Group Defence Council membership, I thank Mr Richardson for his service and wish him and his wife, Betty, a happy retirement together."
Richardson held the Department of Defence secretary position since 18 October 2012.
He was appointed as an Officer of the Order of Australia for service to the community in a range of public policy areas including foreign policy, immigration and security.
Brendan Sargeant, associate secretary, will be acting secretary pending the appointment of Richardson’s successor.