Professor L'Estrange graduated with Honours from Sydney University in 1974, majoring in History. He was awarded the 1975 NSW Rhodes Scholarship and studied at Oxford University from 1976 to 1979.  He graduated with First Class Honours in Philosophy, Politics and Economics and won two University Blues for cricket.

In 1981, Professor L'Estrange joined the Department of Prime Minister and Cabinet.  Among other appointments, he served in 1984-85 on the staff of Mr Justice Hope's Royal Commission into Australia's Security and Intelligence Agencies.

In 1986, Professor L'Estrange was awarded a Harkness Fellowship and spent two academic years under the auspices of the Fellowship studying at the School of Foreign Service at Georgetown University in Washington DC and at the Institute of International Studies at the University of California at Berkeley.

From 1989 to 1994, Professor L'Estrange worked for several Leaders of the Opposition in a range of policy advisory positions.  In 1995, he was appointed the inaugural Executive Director of the Menzies Research Centre in Canberra.

In March 1996, Professor L'Estrange was appointed by the Prime Minister as Secretary to Cabinet and Head of the Cabinet Policy Unit.  He served in that capacity until July 2000 when he became Australia's High Commissioner to the United Kingdom.

Professor L'Estrange returned from that posting in January 2005 to take up the position of Secretary of the Department of Foreign Affairs and Trade in Canberra, a position he held until August 2009.

In 2007, Professor L'Estrange was appointed as an Officer in the Order of Australia 'for service to the development and implementation of public policy in Australia, particularly national security and foreign policy, and to international relations through fostering diplomatic, trade and cultural interests including strengthening Australia's relationship with the United Kingdom'.

Professor L'Estrange was appointed Director of the National Security College at the Australian National University in December 2009.

Professor L'Estrange is married and has five sons.