Sullivan is the Kissinger Professor of the Practice of Statecraft and World Order at the Harvard Kennedy School and a Senior Fellow at the Carsey School of Public Policy at the University of New Hampshire. He was the 28th Assistant to the President for National Security Affairs (National Security Advisor) from 2021 to 2025. In the Obama Administration, he served as National Security Advisor to then-Vice President Biden, Director of the Policy Planning Staff at the U.S. Department of State, and Deputy Chief of Staff to Secretary of State Hillary Clinton. He has held teaching posts at Dartmouth College and Yale Law School.