Beginning in 1948, Henkin worked at the United Nations bureau of the United States Department of State, where he was one of the individuals responsible for the Convention Relating to the Status of Refugees in 1951, an agreement that established the internationally agreed upon definition of what constitutes a refugee and established the requirements for countries to provide asylum to individuals so designated. He left the State department in 1956 to teach for a year at Columbia University on the subject of nuclear disarmament which became the subject matter for his 1958 book "Arms Control and Inspection in American Law". He taught at the University of Pennsylvania starting in 1958, continuing his work that was published as "The Berlin Crisis and the United Nations" in 1959 and the book "Disarmament: The Lawyer's Interests", which was released in 1964.