Kay was born the son of Sir Ector, a nobleman of the Welsh Marches who was entrusted by Merlin with the infant Arthur after Uther Pendragon's death. Ector raised the boy as his own, and Kay grew up believing Arthur was merely a younger foster-brother, the child his father had taken in. The two boys trained together, fought together, and shared every secret, though Kay was the elder and the heir to his father's house. When Arthur drew the sword from the stone and was proclaimed king, Kay was among the first to kneel, and his father asked only one favour: that Arthur make Kay his seneschal, the steward who managed the royal household, the kitchens, the feasts, and the provisions that kept the court alive. Arthur swore that no other man would hold the office while they both lived, and Kay became the most powerful servant in the kingdom, the man who controlled the keys to every storeroom and the roster of every scullion.

In the earliest Welsh tradition, recorded in the medieval tale Culhwch and Olwen of the 11th or 12th century, Kay is a figure of almost supernatural power. He is called Cei, one of the three enchanter knights of Britain, and his abilities are described with wonder: he could hold his breath underwater for nine days and nine nights, he could go without sleep for the same span, a wound from his sword could not be healed by any physician, and when it pleased him he could grow as tall as the tallest tree in the forest. He was the foremost warrior at Arthur's court, and when Culhwch, the hero seeking the giant's daughter Olwen, came to Camelot, it was Cei who led the expedition. This Welsh Cei is a far cry from the seneschal of later romance, a hero of mythic stature who fought monsters and bore the heat that could dry his companions' clothes in the heaviest rain.

The French romances of the 12th century, particularly those of Chretien de Troyes, the poet who invented the Arthurian romance as a literary genre, transformed Kay from the Welsh superman into the sharp-tongued steward who serves as a foil for the great knights. In Chretien's Yvain, the Knight of the Lion, Kay mocks Sir Calogrenant for his chivalric courtesy and provokes him into the adventure that drives the tale. In Chretien's Lancelot, the Knight of the Cart, Kay tricks Arthur into letting him attempt the rescue of Queen Guinevere, the wife of King Arthur, from the abductor Maleagant, and is humiliatingly defeated. In Chretien's Perceval, the Story of the Grail, Kay slaps a maiden who prophesies that Perceval, the naive Welsh boy who would become the greatest Grail knight, will achieve greatness, and Perceval later avenges her by breaking Kay's shoulder. Yet even in these tales, Kay is never a coward or a traitor. His mockery is the test that proves the hero's virtue, and his sharp words maintain the order of the court.

Thomas Malory, in his Le Morte d'Arthur compiled in 1469-1470, gave Kay his most enduring form. Malory's Kay is the seneschal who manages the kitchens and who, when Gareth of Orkney arrived at Camelot in rags asking for food and drink for a year, put him to work among the scullions and mocked him with the name Beaumains. It was Kay who set the kitchen boy to labour, Kay who laughed at his white hands, and Kay who was silenced when Gareth proved himself the conqueror of the Red Knight of the Red Lands. Yet Malory also gives Kay moments of genuine glory. During the invasion of the five kings of Denmark, Ireland, the Vale, Soleise, and the Isle of Longtains, Kay stood with Arthur, Gawain, and Griflet by the Humber River when the enemy attacked by night. Kay saw five kings riding toward them and declared that he would take two of them while the others took the rest. He charged, struck one king through shield and body so that the man fell dead, then ran to the fifth and clove helm and head with a single blow. Arthur praised him worshipfully, and Queen Guinevere herself declared that any lady who did not love Kay was greatly to blame.

Kay's death is told differently in different traditions. In Geoffrey of Monmouth's Historia Regum Britanniae, written around 1138, Kay is killed in the war against the Roman Emperor Lucius, avenged by Arthur who splits the emperor's head to the teeth. The Vulgate Cycle also has him die in France fighting the Romans. In Welsh literature, he is killed by a warrior named Gwyddawg, who is in turn killed by Arthur. But Malory's version is more ambiguous: Kay survives the Roman war, is sent to retrieve the sacred scabbard of Excalibur, Arthur's sword, before the final battle of Camlann, and is mentioned among the few who survive that last terrible conflict. Some later traditions suggest he was killed by Lancelot during the rescue of Guinevere from the stake, confused with Sir Kainus le Strange, but Malory himself leaves Kay's fate uncertain, the foster-brother who may have outlived the kingdom he served.