Mark Simonoff served at the US Mission to the United Nations (UN) from August 2008 to January 2025, most recently as Legal Adviser. In this role, he provided legal counsel to Ambassadors Linda Thomas-Greenfield, Kelly Craft, Nikki Haley, Samantha Power, and Susan Rice. Over the course of his tenure, he negotiated numerous UN Security Council and General Assembly resolutions, including those addressing Ukraine, Syria, crimes against humanity, and the International Criminal Court. He has deep expertise in the Security Council and General Assembly procedures.

Previously, he spent a decade in the Office of the Legal Adviser at the US Department of State, where he focused on a wide range of legal issues involving the Antarctic, Arctic, oceans, outer space, and the environment, among others. He also litigated cases against Iran at the Iran-US Claims Tribunal.

Earlier in his career, he worked as a litigator in both private and government practice in Boston, Massachusetts, and clerked for judges on the US District Court and the Supreme Judicial Court of Massachusetts. 

He holds a law degree from Harvard Law School and a bachelor’s degree from Yale College.

Mr. Simonoff is a Senior Fellow at NYU’s Center on International Cooperation. He is also a Senior Fellow at the American Society of International Law. He is a life member at the Council on Foreign Relations.