Hillen was an officer in the United States Army for twelve years. He has served as a civil affairs officer on jump status with the United States Special Operations Command and as a cavalry officer in the 2nd Cavalry Regiment. He was featured in Douglas Macgregor's military memoir Warriors Rage: The Great Tank Battle of the 73 Easting. While serving as Assistant Secretary of State for Political-Military Affairs, Hillen was the senior State department official responsible for coordinating America's diplomatic strategy with its military operations. He was also responsible for overseeing the department's policies in the areas of international security, security assistance, military operations, weapons removal and abatement, and defense trade. He was unanimously confirmed by the Senate. Prior to his appointment in the Bush administration, he was the head of the defense and intelligence business at American Management Systems. Prior to that, he was the Chief Operating Officer of Island ECN. He is a trustee or director of several non-profit institutions, including the International Institute for Strategic Studies, The Foreign Policy Research Institute, Hampden–Sydney College, and National Review. He was for a contributing editor to National Review and an ABC News consultant. He is also a member of the Council on Foreign Relations and the Veterans of Foreign Wars. Hillen is the author and editor of several books on international security, including Future Visions for U.S. Defense Policy: Four Alternatives Presented as Presidential Speeches–A Council Policy Initiative and Blue Helmets: The Strategy of UN Military Operations. and he has been published in Foreign Affairs, The New York Times, and The Wall Street Journal. From 2007 to 2016, Hillen served from on the Chief of Naval Operations Executive Panel, the advisory panel for the head of the United States Navy. In 2016, he was awarded the Navy Meritorious Public Service Award for service to that panel. Also in 2016, Hillen was awarded the Algernon Sydney Sullivan Award. Hillen was a military advisory for and is featured as a character in the game. In August 2017, Hillen was elected to the new board of advisers for market research provider Govini. Hillen partnered with leading executive coach Mark Nevins to write the book What Happens Now?: Reinvent Yourself as a Leader Before Your Business Outruns in May 2018.
