Filler served as a research historian for the American Council of Learned Societies (1942-1944), and as a historian for the War Department (1944-1946). He joined the faculty of Antioch College, where he was appointed professor of Americancivilization in 1946.H e was afellow of the Social Science Research Council and American Council of Learned Societies (1953-1954). Filler's major work was in the field of American reform movements and cultural developments. Among his works are Crusaders for American Liberalism (1939, 1964), Randolph Bourne (1943), The Crusade against Slavery, 1830-1860 (1960), A Dictionary of American Social Reform (1963), The Unknown Edwin Markham (1967), Appointment at Armageddon: Muckraking & Progressivism in American Life (1976), Vanguards & Followers: Youth in the American Tradition (1978), The Rise & Fall of Slavery in America (1980), A Dictionary of American Social Change (1982), Dictionary of American Conservatism (1987), Distinguished Shades: Americans Whose Lives Live On (1992), The Muckrakers (1993), American Anxieties: ACollective Portrait of the 1930s (1993), Muckraking and Progressivism in the American Tradition (1996), and Slavery in the United States (1998). He taught at Antioch College in Yellow Springs, Ohio, first as professor of American civilization from 1953 until 1976, and then as Distinguished University Professor of American Culture and Society.