St. John was born in the Danube Delta in what is now Dobrogea, Romania, in about 360 (some sources instead place him as a native of Gaul). In 382 he entered a monastery in Bethlehem and after several years there was granted permission, along with his friend St. Germanus of Dobrogea, to visit the Desert Fathers in Egypt. They remained in Egypt until 399, except for a brief period when they returned to Bethlehem and were released from the monastery there.
Upon leaving Egypt they went to Constantinople, where they met St. John Chrysostom, who ordained St. John Cassian as a deacon. He had to leave Constantinople in 403 when Chrysostom was exiled, eventually settling close to Marseilles, where he was ordained priest and founded two monasteries, one for women and one for men.
St. John's most famous works are the Institutes, which detail how to live the monastic life, and the Conferences, which provide details of conversations between John and Germanus and the Desert Fathers. He also warned against some of the excesses in St. Augustine of Hippo's theology whilst refraining from criticising him by name. For this reason he has sometimes been accused of Semi-Pelagianism by the Roman Catholic Church and some Protestant commentators.
St. John died peacefully in 435.
(Biography from OrthodoxWiki)
