ascetic, monk, theologian, and founder and first abbot of the famous abbey of Saint-Victor at Marseille. His writings, which have influenced all Western monasticism, themselves reflect much of the teaching of the hermits of Egypt, the Desert Fathers. Cassian's theology stemmed from, and was subordinate to, his concept of monasticism. He became a leading exponent of, in its early phase, semi-Pelagianism, a heresy that flourished in southern France during the 5th century.

Probably of Roman birth, Cassian became a monk at Bethlehem and later visited and was trained by the hermits and monks of Egypt. About 399 he went to Constantinople, where he was ordained a deacon by the patriarch, St. John Chrysostom (Chrysostom, Saint John). A few years later, after Chrysostom had been illegally deposed, Cassian went to Rome to plead Chrysostom's cause with the pope and while there was ordained a priest (405). Nothing is then known of his life until 415, when he founded a nunnery at Marseille and also the abbey of Saint-Victor, of which he remained abbot until his death.

Cassian's most influential work is his Institutes of the Monastic Life (420–429); this, and his Collations of the Fathers (or Conferences of the Egyptian Monks), written as dialogues of the Desert Fathers, were influential in the further development of Western monasticism. His theological dissertation On the Incarnation of the Lord, written against the heretic Nestorius at the request of Pope Leo I, is an inferior work.