After his release from active service in the United States Navy, he returned to Harvard University for two years and then joined the history faculty of Northwestern University in 1948, where he spent the remainder of his career.
Leopold served on numerous governmental advisory committees, including the Secretary of the Navy's Advisory Committee on Naval History, The Department of State Historical Advisory Committee, The Department of the Army Historical Advisory Committee, Atomic Energy Commission's Historical Advisory Committee, CIA Historical Review Panel and Library of Congress.