Cosmas was an orphan adopted into the wealthy Christian family of John of Damascus's father — the great Syrian Christian Mansur who served as chief logothete (finance minister) under the early Umayyad caliphs. He was raised as foster-brother to John, sharing his tutor Cosmas the Sicilian and his theological formation.
Like John he became a monk at the Lavra of St. Sabbas south of Bethlehem, where the two friends spent decades together composing the great corpus of Byzantine liturgical hymnography that bears their joint signature. Cosmas was the principal composer of the kanon-form hymns for the great Marian and Christological feasts, including the Eve of Christmas, the Hypapante (Presentation), the Annunciation, Lazarus Saturday, and the Sundays of Pentecost.
Reluctantly elected bishop of Maiuma (the port of Gaza) in old age, he governed the see briefly and died around 760. His hymns continue to be sung in every Byzantine and Eastern Catholic liturgy of the feasts they serve.