In 1899, following the death of Arthur Giry (1848–1899), he was appointed professor of diplomatics at the École des Chartes.
In 1916 he became the school's director, a position he maintained until 1930.
Within this time period (1916–1919), he also taught classes on histoire des institutions, filling in for Paul Viollet (1840–1914).
Prou played a major role in the revival of history of law and its institutions during the latter part of the 19th century.