Correspondence, memoranda, diaries, speeches, writings, reports, notes, newspaper clippings, memorabilia, photographs, and other papers relating primarily to Long's career as third assistant secretary of state under Woodrow Wilson (1917-1920), ambassador to Italy (1933-1936), ambassador on special mission to Argentina, Brazil, and Uruguay (1938), and assistant secretary of state under Franklin D. Roosevelt (1940-1944). Also documents Long's work as secretary for the Missouri Code Commission for the revision of judicial procedure (1914), chairman of the National Jefferson Centennial Committee (1925-1929), in the Democratic Party, and trustee of Princeton University, Princeton, N.J. (1937-1941). Includes drafts of Long's Genesis of the Constitution of the United States of America (1926); papers (1740-1945) of the Blair, Breckinridge, Long, and Preston families; material relating to political campaigns (1908-1944); and an autograph collection (1486-1923).

Correspondents include Newton Diehl Baker, Ray Stannard Baker, Bernard M. Baruch, Desha Breckinridge, Jefferson Caffery, Wilbur J. Carr, Bainbridge Colby, Homer S. Cummings, Josephus Daniels, J. Lionberger Davis, James A. Farley, Carter Glass, Harry Bartow Hawes, W.R. Hollister, Edward Mandell House, Andrieus Aristieus Jones, Michael Kinney, Robert Lansing, W.G. McAdoo, Vance Criswell McCormick, Wilbur W. Marsh, George S. Messersmith, George Fort Milton, Henry Morgenthau, A. Mitchell Palmer, William Phillips, Key Pittman, Frank L. Polk, Franklin D. Roosevelt, Daniel C. Roper, Selden P. Spencer, William Stanley, Herbert Bayard Swope, Frank Abner Thompson, Guy Atwood Thompson, Joseph P. Tumulty, and Woodrow Wilson.