Geraint was born into the royal house of Dumnonia, the southwestern kingdom of Britain whose lands stretched across what is now Devon and Cornwall. His father was King Erbin, a figure remembered in Welsh hagiography and genealogy, and his brothers were Ermid and Dywel. Geraint himself was already a celebrated hero before the Arthurian romances ever wrote his name, for a Welsh poem in the Black Book of Carmarthen, composed perhaps as early as the tenth century, sings his praises at the Battle of Llongborth. The poem calls him the bravest of the brave, a man whose deeds were sung by bards long before Chretien de Troyes ever set quill to parchment. The earlier Welsh war poem Y Gododdin, composed around 600 CE, may also allude to a Geraint "before the South," suggesting that the name belonged to a genuine historical warrior of the Arthurian age. Later genealogies associate him with the Dumnonian royal houses of the late sixth century, and a historical King Geraint of Dumnonia is recorded as having been killed in battle against the Saxons in 710 CE, a date that may have become entangled with the legendary figure.

The defining adventure of Geraint's life began at a Whitsuntide feast at Caerleon-on-Usk, where King Arthur held his court. Arthur announced a ceremonial hunt of a white stag, a Celtic motif of great antiquity in which the pursuit of a magical animal leads to an otherworld encounter. Geraint accompanied Queen Gwenhwyfar and her handmaiden into the forest, but the hunting party became separated from the main host. In a clearing they encountered a mysterious knight of supernatural height, clad in outlandish full-body armour, accompanied by a proud lady on a palfrey and an ill-aspected dwarf. The dwarf struck Gwenhwyfar's handmaiden and refused to give his master's name, and Geraint, furious at the insult to the queen's household, swore to avenge the slight. He followed the knight into his domain, and in doing so he found both his destiny and his bride.

Geraint lodged with an old earl named Niwl, called Yniol in some versions, a man whose earldom had been usurped by his own nephew and who lived in poverty with his wife and his beautiful daughter Enid. Geraint was struck by Enid's beauty and her goodness, and he conceived a plan that would serve two purposes at once. He would challenge the mysterious knight, whose name was Edern fab Nudd, at the sparrowhawk tournament that was about to be held, and in winning the tournament he would win Enid's hand. Edern was a figure of deep archaic roots, his patronymic Nudd derived from the Romano-British deity Nodens, and his appearance in the tale places the episode firmly in the oldest stratum of Welsh tradition. The next day Geraint defeated Edern in single combat, winning the sparrowhawk and announcing his intention to marry Enid. He brought her back to Arthur's court, where Queen Gwenhwyfar herself clothed the bride in garments fit for a queen, and King Arthur celebrated the wedding with a feast of such magnificence that lords came from every corner of the realm.

After the wedding, Geraint and Enid travelled to his father's castle in Dumnonia, where King Erbin welcomed his new daughter-in-law. For a time, all was bliss. Geraint and Enid fell into a life of such deep domestic happiness that Geraint spent his days in their chamber rather than on the tournament field. He neglected his knightly duties, forgot the call of adventure, and allowed his sword to rust. Rumours began to spread through the kingdom that the once-great knight had gone soft, that Enid had unmanned him, and that he was no longer fit to be called a warrior. The gossip reached Enid's ears, and one morning, believing Geraint to be asleep, she wept to herself that she was not a true wife, for she had kept her husband from the chivalric duties that were his birthright. Geraint was not asleep. He heard her words, but he heard only the part about her not being a true wife, and in his wounded pride he misunderstood her lament as a confession of adultery. He told her to prepare for a journey, and without explanation he led her out of the castle and onto the open road, commanding her to ride ahead of him and never to speak to him unless he spoke first.

The journey became a test of both their souls. Enid, though forbidden to speak, could not keep silent when she saw danger approaching. Again and again she disobeyed his command, warning him of bandits lurking in the woods, of armed knights waiting by the road, of giants camped in the hollows. Each time she spoke, Geraint rebuked her for breaking her promise, and each time he fought the danger she had warned him of, proving with every clash of steel that he was still the warrior he had always been. In one terrible episode they came to the castle of Earl Limwris, a man who offered them hospitality and then tried to force himself upon Enid while Geraint lay unconscious from wounds he had taken in a battle with three giants. Enid screamed, and the sound brought Geraint back from the edge of death. He rose, seized his sword, and killed the earl where he stood. In another adventure, in the Welsh version of the tale, they encountered the Hedge of Mist, a magical barrier behind which a knight named Mabonagrain had imprisoned himself and his lover in a cycle of endless combat, a device that echoed the otherworld prisons of Welsh mythology. Geraint defeated Mabonagrain and freed the lovers, proving that he could break the enchantments that held others captive even while he was himself captive to his own jealousy.

At last, after countless battles and endless miles of road, Geraint could deny the truth no longer. Enid had disobeyed him not out of defiance but out of love. She had risked his wrath, his sword, and his contempt to save his life, and in doing so she had proven her fidelity a hundred times over. Geraint asked for her forgiveness, and she gave it without hesitation. The two were reconciled, and they returned to Dumnonia together, no longer a jealous husband and a suspected wife but a knight and his lady who had survived the fire of doubt and emerged whole. Geraint resumed his place as the foremost warrior of his father's realm, and when King Erbin grew infirm, Geraint inherited the crown of Dumnonia. He ruled with justice and vigour, keeping the peace of King Arthur's realm and proving himself in joust and tournament once more.

The end of Geraint's story varies with the teller. In the early Welsh poem, he died at the Battle of Llongborth, perhaps at Langport on the River Parrett in Somerset, fighting against the Anglo-Saxon invaders who were pressing ever westward into Dumnonian territory. The poem calls him the bravest of the brave, and his followers are described as "brave men from the land of Devon." In Alfred Tennyson's Idylls of the King, written in the Victorian age and based on Lady Charlotte Guest's translation of the Welsh Mabinogion, Geraint dies fighting heathens on the Northern Sea, a death that Tennyson invented to give the tale a heroic conclusion. In the French version by Chretien de Troyes, where Geraint is called Erec, the hero does not die at all but lives out his days in peace with Enide, having learned to balance the demands of love and the demands of honour. The Welsh romance, preserved in the White Book of Rhydderch and the Red Book of Hergest, ends with Geraint inheriting his father's kingdom and ruling in happiness, the jealousy that had nearly destroyed him left behind on the road like a shed skin.