He specializes in the history of American community formation in the eighteenth century. His first book was Mammon and Manon in Early New Orleans: The First Slave Society in the Deep South, 1718-1819. It describes the development of New Orleans Parish over its first century under French, Spanish, and finally American republican administration. It encompasses the entwined histories of the black slave majority and the various groups of free people, and seeks to explain how this town became the most distinctive city in the United States.

In his new work, "To Intermix With Our White Brothers": Indian 'Mixed Bloods' in the United States from Earliest Times to the Removals of the 1830s, the argument is that Indians of mixed ancestry identified with their Native American tribes of birth or adoption, although most were also willing to mix freely with whites. Some of them provided tribal leadership in resisting the federal policy of Indian removal, which became law in 1830, a law white leaders intended, in part, to prevent more racial mixture of the kind the mixed bloods represented.

Professor Ingersoll is currently researching a work on New England towns during the American Revolution.

Ingersoll has published articles in William and Mary Quarterly, Law and History Review, and other journals.

He teaches specialized courses on African Americans, Early American Radicalism, the American Revolution, and the Civil War.