In 1940, Kirk was appointed to the faculty of Columbia University as an associate professor of government. He was promoted to full professor in 1943 and began a long association with the U.S. government when he served in the Security Section of the United States Department of State's Political Studies Division during World War II. Kirk became involved in the formation of the United Nations Security Council, attending the Dumbarton Oaks Conference and the United Nations Conference on International Organization where the United Nations Charter was signed.