He held that post until 1928, at which time he joined the faculty of the Union Theological Seminary, New York City, where he taught for 30 years. At the time of his retirement (1960) he held a chair of ethics and theology; he also served as dean (1950-55) and vice president (1955-60). After retiring he continued at Union as a lecturer. An outstanding, although not a systematic, theologian, Niebuhr was notable primarily for his examination of the interrelationships between religion, individuals, and modern society.
Outside the field of theology, he took a keen interest in trade union and political affairs. He was an active member of the Socialist Party in the 1930s, waged a vigorous fight against isolationism and pacifism before and during World War II, and in 1944 helped to found the Liberal Party in New York State. He received the U. S. Presidential Medal of Freedom in 1964 and was made a member of the American Academy of Arts and Letters.
He died on June 1, 1971. Niebuhr indicated his overriding interest in what has been called theological anthropology, a concern with the nature of man as a contact point for religion and society, in such major works as Moral Man and Immoral Society (1932), Interpretation of Christian Ethics (1935), and The Nature and Destiny of Man (2 volumes, 1941, 1943).
A penetrating critic of society, he also published Faith and History (1949), Christian Realism and Political Problems (1953), The Self and the Dramas of History (1955), and Structure of Nations and Empires (1959). In addition he edited Christianity and Society, a quarterly, and the biweekly periodical Christianity and Crisis.