Bishop of Vienne, date of birth unknown; died shortly after 475. Concerning the life of Mamertus before his elevation to the See of Vienne, nothing certain is known. The fact that his brother, Claudianus Mamertus, the theological writer, received in his youth a sound training in rhetoric, and enjoyed the personal acquaintance of Bishop Eucherius of Lyons (434-50), suggests that the brothers belonged to a wealthy Gallic family from the neighbourhood of Lyons.
Like his brother, St. Mamertus was distinguished for his knowledge of profane subjects as well as of theology, and, before his elevation to the episcopate, appears to have been married. His election and consecration took place shortly before 462. As bishop he enlisted the services of his brother, who had withdrawn to a cloister, and ordained him priest of Vienne. The activity of the brothers is described in a letter of Sidonius Apollinaris, another of whose letters is addressed to Bishop Mamertus. In 463 Mamertus was engaged in a dispute with Pope Hilarius on the question of the privileges of the Bishop of Arles. Mamertus evidently submitted, since we find no subsequent reference to the incident.
St. Mamertus is best remembered as he originator of the penitential practice of Rogation days. This practice is marked by processions and Psalms for the three days preceding the feast of the Ascension.