His most significant work has dealt with the relationships among elites and laymen in medieval life, particularly in the realm of religious culture, where he has focused on ideas and topics such as superstition, the occult and heresy in order to flesh out the differing world-views of the lay peasantry and the clerical elites who attempted to define religious practice.
He has also written widely on the cult of saints, the idea of adolescence, visions and dreams, and preaching.
Among Schmitt's best known works translated in English are The Holy Greyhound (1983), about the strange cult of a holy dog in medieval France, and Ghosts in the Middle Ages (1998) about notions of death, the afterlife and paranormal visions in medieval culture.
Schmitt has argued that this has helped correct for the tendency among medievalists in the past to focus on elites, political institutions and narrative history to the exclusion of the lower classes and their less well-documented experiences of life.
Until 2014 Schmitt was Director of Studies at the École des Hautes Études en Sciences Sociales and directed the society of professional historians, Groupe d'Anthropologie Historique de l'Occident Médiéval.