Geoffrey was born around 1095, probably in Monmouth, a town on the Welsh Marches where the River Wye meets the River Monnow, a place where Norman, Welsh, and Breton cultures mingled in the decades after the Conquest. His name appears on charters as "Galfridus Monemutensis," Geoffrey of Monmouth, and scholars believe he may have been of Breton descent, his family perhaps arriving with William the Conqueror's forces from Brittany, the Celtic homeland that shared Wales's language and legends. He was educated, possibly at the priory of St Mary's Church in Monmouth, and by 1129 he had appeared in Oxford, where his name appears on the charter of Osney Abbey. For the next twenty years, he was an Oxford cleric, linked with Walter, Archdeacon of Oxford, a man who shared his taste for letters and who, according to Geoffrey's own prologue, presented him with the "very ancient book in the British tongue" that became the supposed source of the Historia. Whether this book existed or was a literary fiction, the claim was a common medieval device for authenticating a new work, and Geoffrey used it to bridge the gap between the legendary past and the Norman present.

The Prophetiae Merlini, composed around 1135 with the encouragement of Alexander, Bishop of Lincoln, was Geoffrey's first Arthurian work. It was a collection of cryptic prophecies attributed to Merlin, the boy-prophet who had revealed the red and white dragons to Vortigern, and it enjoyed immense popularity as political prophecy. Sections were translated into Middle English and used to signify political events for centuries, and the work established Merlin as a figure of national destiny rather than mere local legend. But it was the Historia Regum Britanniae, completed by 1138, that changed everything. In twelve books, Geoffrey traced the history of Britain from the arrival of Brutus, the great-grandson of Aeneas, who founded Britain after the Trojan War, through the reigns of King Lear, King Cymbeline, and Old King Cole, to the rise and fall of King Arthur. He gave Arthur a father, Uther Pendragon, who fell in love with Igraine through Merlin's enchantments. He gave Arthur a sword, Caliburn, forged in Avalon. He gave Arthur a queen, Guinevere, who was descended from a noble Roman family. He gave Arthur a son, Mordred, born of the incestuous union between Arthur and his half-sister Morgause. He gave Arthur a Round Table, a fellowship of knights, and a conquest of Rome that rivalled the deeds of Alexander the Great. And he gave Arthur a death: wounded at the battle of Camlann, carried to the Isle of Avalon to be healed of his wounds, leaving the kingdom to fall to the Saxons. None of this had existed in a single narrative before Geoffrey wrote it.

Geoffrey's sources were a patchwork of earlier texts and oral traditions. He drew upon Gildas's De Excidio Brittaniae, a 6th-century lament for the ruin of Britain, which mentioned Vortigern and Aurelius Ambrosius. He drew upon Nennius's Historia Brittonum, a 9th-century chronicle that first mentioned "Arthur the soldier," the dux bellorum who fought twelve battles against the Saxons. He drew upon Bede's Ecclesiastical History of the English People, Welsh genealogical tracts, regnal lists, and the rich oral folklore of Wales and Brittany. But he also invented freely, filling gaps with his own imagination, and the result was a work that his contemporaries immediately recognised as something between history and fiction. William of Newburgh, a 12th-century Augustinian canon, lambasted the Historia as "a pack of lies" and accused Geoffrey of shamelessly inventing stories about the Britons. But the book was a bestseller. Two hundred and seventeen surviving manuscripts testify to its popularity, and it was translated into Norman French by Wace around 1155, into Middle English by Layamon around 1200, and into Welsh, Cornish, and other languages. It became the foundation for the entire Arthurian tradition, the source from which Chrétien de Troyes drew his romances, from which Malory composed his Morte d'Arthur, and from which Tennyson and every modern reteller has taken their Arthur.

The political context of Geoffrey's work was the Norman conquest and the struggle for legitimacy. The Anglo-Norman kings needed a history that connected them to the pre-Saxon British past, a lineage that justified their rule over Wales and the Welsh Marches. Geoffrey's Historia provided exactly that: a narrative in which the Britons, ancestors of the Welsh, had once ruled a vast empire that included Rome itself, and in which the Saxons were usurpers who had stolen the land from its rightful heirs. The Norman kings, by this logic, were the restorers of British glory, the new Arthurs who would reclaim the island from the Saxon descendants. The dedication of the Historia to Robert, Earl of Gloucester, the illegitimate son of Henry I and one of the most powerful magnates in England, was a political masterstroke, securing patronage and protection for a work that might otherwise have been condemned as heretical or treasonous. Geoffrey's later appointment as Bishop of St Asaph in 1152, consecrated by Archbishop Theobald of Bec at Lambeth, was a reward for his literary service, though there is no evidence that he ever visited his Welsh see, which was in the midst of unrest.

In his final years, Geoffrey returned to the character who had made him famous. Around 1150, he composed the Vita Merlini, a poem of 1,530 lines in Latin hexameters that portrayed Merlin in a very different light from the boy-prophet of the Historia. This Merlin was a king of Dyfed who went mad after the slaughter of his companions in battle, fled to the Caledonian forest, and lived as a wild man among the beasts. He was restored to sanity by the music of a messenger and the love of his sister Ganieda, but he chose to return to the woods, preferring the company of nature to the courts of men. The poem introduced Taliesin, the legendary Welsh poet, as Merlin's companion, and it explored themes of madness, prophecy, and the rejection of civilisation that would resonate through later literature. Geoffrey died in 1155, leaving behind a body of work that had transformed the cultural landscape of Europe. Within a generation, Wace had added the Round Table to Arthur's court, Chrétien de Troyes had invented Lancelot and the Grail Quest, and the Arthurian legend had become the Matter of Britain, the greatest secular mythology of the Middle Ages. Geoffrey of Monmouth had not merely written about Arthur; he had created him, breathing life into a figure who had been little more than a name in a chronicle and turning him into the once and future king, the symbol of British identity, and the hero of the greatest story ever told.