This essay examines the career of William E. Leuchtenburg, one of the most important US historians since 1945. It is the fullest published account of this major historian and makes extensive use of his correspondence with mentors, colleagues and students, including Henry Steele Commager, Richard Hofstadter, Howard Zinn, Christopher Lasch, and Arthur M. Schlesinger, Jr. It argues that generational differences do not explain the ease with which Leuchtenburg’s moved in what he called ‘the public realm’. More important, I suggest, is the character of post-war American liberalism and its relationship to the institutional setting of the modern university.