After graduating, he served as a law clerk to Judge Learned Hand of the United States Court of Appeals for the Second Circuit.
[1] Henkin enlisted in the United States Army in June 1941 and saw action during World War II in the European Theater in Sicily, Italy, France and Germany.
After completing his military service, he was a law clerk for Supreme Court Associate Justice Felix Frankfurter.
[5] 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 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.
[1] Written while Richard Nixon was conducting the American involvement in the Vietnam War, his 1972 book Foreign Affairs and the Constitution described the division of responsibility between the President of the United States and the Congress in conducting foreign affairs and military action, exploring how the executive branch had achieved a great measure of control despite the fact that the Constitution grants the legislative branch the power to declare war.