All the information we have about David is based on the unreliable eleventh-century biography written by Rhygyfarch, the son of Bishop Sulien of St. David's. According to it David was the son of King Sant of South Wales and St. Non, became a priest, studied under St. Paulinus on an unidentified island for several years, and then engaged in missionary activities, founding some dozen monasteries, the last of which, at Mynyw (Menevia) in southwestern Wales, was noted for the extreme asceticism of its rule, which was based on that of the Egyptian monks. David attended a synod at Brefi, Cardiganshire, in about 550 where his eloquence is said to have caused him to be elected primate of the Cambrian Church with the understanding that the episcopal see would be moved from Caerleon to Mynyw, now St. David's. He was supposedly consecrated archbishop by the patriarch of Jerusalem while on pilgrimage to the Holy Land, and a council he convened, called the Synod of Victory because it marked the final demise of Pelagianism, ratified the edicts of Brefi, and drew up regulations for the British Church. He died at his monastery at Mynyw, and his cult was reputedly approved by Pope Callistus II about 1120. Even his birth and death dates are uncertain, ranging from c. 454 to 520 for the former and from 560 to 601 for the latter.
Excerpted from Dictionary of Saints, John J. Delaney