The historical David is almost irrecoverable behind the layers of Welsh hagiographic tradition. Rhygyfarch's Life of St. David, written around 1094 to defend the independence of the Welsh church against Norman encroachments, is the earliest surviving biography and was composed roughly five hundred years after David's death.

According to Rhygyfarch, David was born the son of a chieftain named Sant and a woman named Non. He was educated at a monastery and became a priest, then founded a monastic community at Menevia (Mynyw) on the western tip of Pembrokeshire. The community practiced an extreme form of Celtic monasticism: the monks ate only bread, vegetables, and salt with water; they performed heavy manual labor; they spent their evenings in prayer, reading, and writing; and they used no animals for agricultural work.

David is traditionally said to have attended the Synod of Brefi, where he preached against Pelagianism — the ground reportedly rose beneath him so that all could hear — and was subsequently acknowledged as archbishop of Wales. He is credited with founding or inspiring numerous churches across South Wales and beyond.

David died around 601. His last words, according to Rhygyfarch, were 'Be joyful, brothers and sisters. Keep your faith, and do the little things that you have seen and heard with me.' The phrase 'Gwnewch y pethau bychain' — 'Do the little things' — has become the most famous saying associated with any Welsh saint.

David is the subject of the most extensive Welsh hagiographic tradition. Rhygyfarch's Life records miracles, divine visions, and spiritual teachings. David is presented as extraordinarily ascetic, possessing miraculous powers, establishing monasteries with miraculous speed, and converting pagan populations. He is credited with pilgrimage to Rome and papal recognition, and with various miracles of healing and divine intervention.