From 2012 to 2017, Dennis served as the Secretary of the Department of Defence, following almost three years as the Secretary of the Department of Foreign Affairs and Trade. 

From 2005 to 2010, Dennis was Ambassador of Australia to the United States. Prior to this, he was Director – General of the Australian Security Intelligence Organisation. 

Dennis has engaged in various senior public service roles in the Departments of the Prime Minister and Cabinet, Foreign Affairs, Trade and Immigration. 

He was also Head of the Review of the Intelligence Community post – Cold War in 1992 and Principal Adviser to the Prime Minister from 1990 to 1991. 

Dennis is a member of the Linfox Board, and US Studies Centre Board of Directors, and holds a Bachelor of Arts (Honours) from the University of Sydney. 

He was made an Officer of the Order of Australia (AO) in 2003 and was advanced to a Companion of the Order of Australia (AC) in 2019.