Duncan joined the Australian Army after schooling in Western Australia, and graduated from the Royal Military College Duntroon, in 1975. A career in the military, spanning 33 years, followed. Duncan commanded at all levels, including command of the Special Air Service Regiment. He served with the United Nations during the 1982 Lebanon War and on operations in East Timor. His last military appointment was as a Major General, Special Operations Commander Australia, where his forces were engaged in operations in Iraq and Afghanistan.
In 2005, Duncan joined the Australian Public Service. His initial appointment was as First Assistant Secretary of the National Security Division within the Department of the Prime Minister and Cabinet. Later that year, he was promoted to Deputy Secretary and in 2008 he was appointed as an Associate Secretary, where he led the National Security and International Policy Group and contributed to the Department’s broader leadership team as a member of its executive.
From December 2008, Duncan served as Australia’s inaugural National Security Adviser. Duncan was the Prime Minister’s principal source of advice on all national security matters. He had responsibility for the strategic leadership of the national security community, as well as the coordination of national security policy development and crisis response. During this period he chaired the National Intelligence Coordination Committee.
In 2011, building on almost four decades of experience in the defence and national security communities, Duncan was appointed Secretary of the Department of Defence.
Late in 2012, Duncan was appointed Ambassador to Belgium, Luxembourg, the European Union and NATO. On 15 September 2014, Duncan was appointed Director- General of Security and head of the Australian Security Intelligence Organisation.
Duncan was awarded Officer of the Order of Australia in 2005, for his service as the inaugural Special Operations Commander Australia.
He holds a Bachelor of Arts degree from the University of NSW and a Graduate Diploma in Defence Studies and Management, from Deakin University. He is a graduate of the British Army Staff College and the United States Army War College. He is fluent in Indonesian.