In early youth he entered the abbey of Lérins then presided over by his kinsman Honoratus (Saint Honoré), and succeeded Honoratus in the bishopric of Arles in 429. Following the example of St Augustine, he is said to have organized his cathedral clergy into a "congregation," devoting a great part of their time to social exercises of ascetic religion. He held the rank of metropolitan of Vienne and Narbonne, and attempted to exercise the sort of primacy over the church of south Gaul, which seemed implied in the vicariate granted to his predecessor Patroclus of Arles (417).
Hilarius deposed the bishop of Besançon, Chelidonus, for ignoring this primacy, and for claiming a metropolitan dignity for Besançon. An appeal was made to Rome, and Pope Leo I used it to extinguish the Gallican vicariate (AD. 444). Hilarius was deprived of his rights to consecrate bishops, call synods, or oversee the church in the province, and the pope secured the edict of Valentinian III, so important in the history of the Gallican church, "ut episcopis Gallicanis omnibusque pro lege esset quidquid apostolicae sedis auctoritas sanxisset." The papal claims were made imperial law, and violation of them subject to legal penalties (Novellae Valent. iii. tit. 16).
Hilarius died in 449, and his name was later introduced into the Roman martyrology for commemoration on May 5. During his lifetime he had a great reputation for learning and eloquence as well as for piety; his extant works (Vita S. Honorati Arelatensis episcopi and Metrum in Genesin) compare favourably with any similar literary productions of that period.
A poem, De providentia, usually included among the writings of Prosper of Aquitaine, is sometimes attributed to Hilary of Arles.