D. Stephen Mathias has served as Assistant Secretary-General for Legal Affairs of the United Nations since 1 September 2010. In this position, Mr. Mathias is the head of the Office of the Legal Counsel and assists 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 has taught as an adjunct professor at Georgetown University Law Center.