Cassian was born around 360, probably in the Roman province of Scythia Minor (modern Romania), though some scholars place his birth in Gaul. As a young man, he entered a monastery in Bethlehem, but the attraction of the Egyptian desert proved irresistible. He and his companion Germanus spent approximately fifteen years among the monks of lower Egypt, receiving instruction from the great abbas — Abba Moses, Abba Isaac, Abba Serenus, and others whose teaching Cassian would later record.

The Egyptian experience was transformative. Cassian encountered men who had spent decades in prayer and ascetical discipline, and whose teaching on the spiritual life was grounded in long experience rather than academic theology. The desert fathers taught him about the eight principal faults (the ancestors of the later seven deadly sins), the stages of spiritual progress, the practice of unceasing prayer, and the subtle operations of grace and human effort in the soul's transformation.

After leaving Egypt, Cassian traveled to Constantinople, where he was ordained a deacon by John Chrysostom. When Chrysostom was deposed and exiled, Cassian went to Rome to plead his cause before Pope Innocent I. He eventually settled in Marseilles around 415, where he founded the monastery of St. Victor for men and a convent for women.

In Marseilles, Cassian wrote his two great works. The Institutes describes the external organization of monastic life — clothing, prayer hours, and the eight principal faults. The Conferences presents the spiritual teaching of the Egyptian fathers in the form of dialogues — twenty-four conversations covering prayer, discretion, spiritual warfare, the goals of monastic life, and the relationship between grace and free will.

This last topic drew Cassian into controversy. His Conference 13, on the will's role in responding to grace, was criticized by Prosper of Aquitaine as insufficiently Augustinian. The later label 'Semi-Pelagianism' is misleading — Cassian affirmed the necessity of grace — but the controversy shadowed his reception in the West. His practical spiritual teaching, however, was universally admired.

Cassian was venerated as a saint and spiritual father by both Eastern and Western traditions from soon after his death. Byzantine and Latin hagiographies developed legends of ascetic feats.