D. Stephen Mathias served as Assistant Secretary-General for Legal Affairs of the United Nations from September 1, 2010 through September 30, 2025. In this position, Mr. Mathias was the head of the Office of the Legal Counsel and assisted in the overall supervision of each of the units of the Office of Legal Affairs.

Before joining the United Nations, he served at the United States State Department in the Office of the Legal Adviser and in the United States Embassy in The Hague. He has also served as General Counsel to the Multinational Force and Observers, which supervises the implementation of the Security Annex to the Peace Treaty between Egypt and Israel. Before joining the public sector, he worked in private legal practice.

Mr. Mathias previously taught as an adjunct professor at Georgetown University Law Center.