He has written on the South, slavery and emancipation, the Populist Era, rural cultures, and social migration. His first book was The Roots of Southern Populism: Yeoman Farmers and the Transformation of the Georgia Upcountry, 1850–1890 (Oxford University Press, 1983). This study was important because it provided a detailed and original account of the political ideology of white southern small farmers. At the time this group, the majority of the American South, had received relatively little scholarly attention. Hahn presented the southern yeomen as non-capitalist in crucial respects, and describes how they were undermined by the increasing commercialization of Southern agriculture after the Civil War. Populists were presented as having had almost no interest in a genuinely biracial polity.
In 2003 Hahn published his second book A Nation Under Our Feet: Black Political Struggles in the Rural South from Slavery to the Great Migration, which won the 2004 Pulitzer Prize for History. Several historians have noted that in this book a union of black and white workers is presented as a much more likely possibility. In 2009 he published The Political Worlds of Slavery and Freedom, a version of the Nathan I. Huggins Lecture he delivered at Harvard University two years earlier. His latest book, A Nation Without Borders: The United States and Its World in an Age of Civil Wars was published in 2016 by Penguin Press.
Hahn has taught at the University of Delaware, the University of California, San Diego, Northwestern University, the University of Pennsylvania, and New York University. He has two children, Declan Hahn and Saoirse, and lives in New York City.
