According to the Roman Martyrology, when he had completed his education in Gaul, Rusticus went to Rome, where he soon gained a reputation as a public speaker, but he wished to embrace the contemplative life. He wrote to Jerome, who advised him to continue his studies, commending him to imitate the virtues of St. Exuperius of Toulouse and to follow the advice of Procule, then Bishop of Marseille.
Thus Rusticus entered the monastery of St. Vincent of Lérins. He was ordained at Marseilles, and on October 3, 430 (or 427) was consecrated Bishop of Narbonne. He was present at the Council of Ephesus in 431 With all his zeal, he could not prevent the progress of the Arian heresy which the Goths were spreading abroad; there is evidence that an Arian rival bishop was established in Narbonne.
Rusticus then endeavored to consolidate the Catholics. In 444-448, he rebuilt the church in Narbonne dedicated to Saint Genès of Arles, which had burned in 441; in 451, he assisted at the convocation of forty-four bishops of Gaul and approved St. Leo's letter to Flavian, concerning Nestorianism; he was present also at a Council of Arles, with thirteen bishops, to decide the debate between Theodore, Bishop of Fréjus, and the Abbey of Lérins. He was one of the twelve bishops who assembled to elect Ravennius bishop of Arles in 449; a letter from Ravennius to Rusticus, proves the high esteem in which he was held. Rusticus' own letters are lost, with the exception of the one to St. Jerome and two others to St. Leo, written either in 452 or 458.