Gildas was born around the year 500, in the same year that the Battle of Mount Badon was fought, the decisive engagement that halted the Saxon advance and gave Britain a generation of peace. He came from the banks of the River Clyde in Strathclyde, the northern kingdom of the Britons that would one day become Scotland, though some later traditions placed his birth in Powys in Wales. He was educated at a monastery, probably Llantwit Major in Glamorgan, under the tutelage of Illtud, a teacher so renowned that Gildas himself called him the educator of all Britain. The world into which Gildas was born was a world in ruins: the Roman legions had departed a century earlier, the cities were crumbling, and the Saxon invaders were pressing westward from the eastern shores. Yet Gildas was trained in the classical tradition, fluent in Latin, steeped in the Bible, and possessed of a literary style that would make his work the most influential text written in Britain for five hundred years.

His masterpiece, De Excidio et Conquestu Britanniae, was written sometime between 516 and 547, probably in the 540s. It is not a history in the modern sense but a sermon, a thunderous religious polemic that uses the past to condemn the present. Gildas divides his work into three parts. The first is a narrative of Roman Britain from the conquest to his own day, a tale of invasion, rebellion, persecution, and divine punishment that includes the first written mention of the Battle of Mount Badon and the hero Ambrosius Aurelianus, the Romano-British leader who rallied the Britons against the Saxons. The second part is a savage denunciation of five British kings, whom Gildas compares to the beasts of the Apocalypse: Constantine the tyrannical whelp of the unclean lioness of Dumnonia, Aurelius Conanus the lion's whelp, Vortiporus the spotted leopard of the Demetians, Cuneglas the bear, and Maglocune the dragon of the island. Each king is torn apart with biblical rhetoric, his sins laid bare for the world to read. The third part attacks the British clergy, whom Gildas calls fools, shameless, and wily plunderers, ministers who had betrayed their sacred trust. The work is a cry of anguish, a lament for a fallen people, and a warning that the Saxons were God's punishment for British sin.

The most famous thing about Gildas is what he did not write. In a work that covers the entire history of post-Roman Britain, that mentions Ambrosius Aurelianus and the Battle of Mount Badon, that condemns kings and bishops by name, there is no mention of Arthur. This silence has haunted historians for centuries. Some have argued that Arthur was not yet famous when Gildas wrote, a minor warlord beneath the notice of a monk who cared only for the great and the sinful. Others have suggested that Gildas deliberately omitted Arthur because of a personal grievance: in the Welsh tradition recorded by Caradoc of Llancarfan in the twelfth century, Gildas was the son of Caw, and his brother Hueil ap Caw was killed by King Arthur. Gildas, preaching in Ireland at the time, was said to have been grief-stricken, but he later forgave Arthur, who performed great penance for the slaying. The tradition grew so strong that by the thirteenth century, poets claimed Gildas had thrown all his books about Arthur into the sea in rage. Whether the omission was deliberate or simply a matter of perspective, the silence of Gildas is the single most important fact in the historical debate over Arthur's existence.

Gildas's later life took him far from Britain. The Annales Cambriae record that he visited Ireland in 565, where he preached and restored church order at the request of King Ainmuire. He then made a pilgrimage to Rome and continued to Brittany, where he founded the monastery of St Gildas-de-Rhuys on a peninsula jutting into the Atlantic. There he lived as a teacher and hermit, and there he died around 570. His body was laid in a boat upon the river and allowed to drift downstream, as he had requested. Three months later, fishermen found the boat and the body intact, a miracle that confirmed his sanctity and made him one of the most revered saints in Brittany. His relics were venerated at Rhuys for generations before being transferred to Berry in the tenth century.

In the Arthurian legends that grew up centuries after his death, Gildas became a character in the very story he had refused to tell. Caradoc of Llancarfan's Life of Gildas, written around 1130, describes how Gildas and the Abbot of Glastonbury persuaded King Melwas of the Summer Region to release Guinevere, whom Melwas had kidnapped and brought to his stronghold at Glastonbury. Arthur had gathered an army to besiege the stronghold, but Gildas's intervention prevented bloodshed and secured the queen's safe return. In this legend, Gildas is the peacemaker, the monk whose holiness could turn the wrath of kings. In Thomas Hughes's Elizabethan tragedy The Misfortunes of Arthur, Gildas appears on stage to lament the state of Britain after Arthur's death, a voice of prophecy and mourning. The historical Gildas, who wrote with such fury about the sins of his own age, would have recognised the role: he was always a man who spoke truth to power, who saw the ruin of Britain and refused to look away, and whose words outlasted every king he condemned.