Henkin is a 1937 graduate of Yeshiva College and a class of 1940 LL.B. graduate of Harvard Law School. He served as a law clerk for Supreme Court Justice Felix Frankfurter (on his difficulties in Sabbath observance when clerking for the Supreme Court, see the story here) and has worked with the U.S. Department of State, NATO and the United Nations. He has published several books, among which figure Foreign affairs and the United States Constitution, The Rights of Man Today, How Nations Behave, and Age of Rights.

His father was Rabbi Yosef Eliyahu Henkin, an authority in halakhah. His son, Joshua Henkin, is a novelist. His second son, David Henkin is a professor of American history at the University of California at Berkeley.