He is known among other things for a biography of Philip I of France and another one of Pope Gregory VII with his reforms.
He also wrote overviews of the history of the Middle Ages.
From 1935, he directed with Victor Martin a Histoire de l'Église depuis les origines jusqu'à nos jours published by Bloud and Gay.
In 1941, he was elected a member of the Académie des Inscriptions et Belles-Lettres.
He was opposed to both the educational theories of Marc Bloch and Bloch personally, probably, suggests Eugen Weber, due to Fliche's innate antisemitism and the fact that Bloch had once given one of Fliche's pieces a poor review.