A Christian humanist in outlook, his work was primarily in the spheres of Late Antiquity and the history of education.
He also edited, for Sources Chrétiennes, the early Christian work Letter to Diognetus, the only manuscript of which perished in a fire at the University of Strasbourg during the Franco-Prussian War.
Marrou edited the collection Patristica Sorbonensia, published by Le Seuil.
His Carnets posthumes were published in 2006 under the editorial supervision of his daughter Françoise Marrou-Flamant.
He was elected a foreign member of the Royal Netherlands Academy of Arts and Sciences in 1967.