Celeus ruled Eleusis, a city on the coast of Attica some twenty kilometres northwest of Athens, in the age when the gods still walked among mortals in disguise. He was married to Metaneira, a queen of gentle heart, and together they had several children. The most famous were their daughters, four princesses who went each day to the Maiden Well to draw water for their household. They also had at least two sons: Triptolemus, the elder, and Demophon, an infant still in the cradle. Celeus was not a warrior king. He did not lead armies or found cities. He was a man of hospitality, and in the Greek world, that was a sacred duty. Zeus himself was called Xenios, the god of guests, and to refuse a stranger was to risk divine punishment.

The stranger who came to Eleusis was Demeter, the goddess of grain and harvest, though no one knew it. Her daughter Persephone had been seized by Hades, the king of the underworld, and carried down to his dark kingdom while she gathered flowers in a meadow. For nine days Demeter wandered the earth with torches, refusing all food and drink, her grief so terrible that the crops withered and no seed would sprout. On the tenth day she came to Eleusis, disguised as an old woman with grey hair and a bent back, and sat down by the Maiden Well, weeping. The four daughters of Celeus found her there. Callidice, the eldest, spoke to her with kindness, telling her that the people of Eleusis were hospitable and that she would find welcome in the house of any nobleman, including their own father. They led her back to the palace, and Celeus and Metaneira received her as a guest.

Demeter, still concealing her divinity, asked for work. She said she was a nurse, experienced in caring for children, and she offered to look after the infant Demophon. Metaneira, grateful for the help, accepted. What followed was one of the most extraordinary episodes in Greek myth. Each day, Demeter anointed the baby with ambrosia, the food of the gods, and breathed her divine breath upon him. Each night, when the household slept, she placed the child in the hearth fire, burning away his mortal flesh bit by bit so that he might become immortal, a god among men. The baby grew with miraculous speed, more beautiful and strong than any mortal child. But one night, Metaneira could not sleep. She crept to the hearth and saw the old nurse holding her son in the flames. She screamed. The scream shattered the spell. Demeter, enraged, pulled the child from the fire and revealed herself in her true form. The palace filled with light and fragrance. The goddess stood before them in all her majesty, and she rebuked Metaneira for her folly. Demophon, she declared, could no longer be made immortal. His mortal flesh had been interrupted at a critical moment, and he would remain a man, though one blessed by the goddess's touch. In some later traditions, the shock killed the child outright, and Demeter gave Triptolemus a chariot of winged dragons as compensation for the loss of his brother.

But Demeter did not destroy Eleusis. Instead, she commanded Celeus to gather the people and build her a temple on the hill above the Maiden Well, with an altar beneath it. Celeus obeyed. He called the assembly and told them what the goddess required. The temple was built, and Demeter withdrew into it, still grieving for Persephone, and refused to let the grain grow. The famine that followed was absolute. The earth produced nothing. Humanity faced extinction, and the gods faced the loss of their sacrifices. Zeus, the king of the gods, was forced to negotiate. He sent Hermes, the messenger with the golden wand, to the underworld to demand Persephone's return. Hades agreed, but before she left, he tricked her into eating pomegranate seeds, binding her to return to him for part of every year. The compromise restored the seasons: spring and summer when Persephone was with her mother, autumn and winter when she returned to the underworld.

When the crisis was over and the earth grew green again, Demeter turned to Celeus and his family with gratitude. She taught the king the conduct of her rites. She initiated him into the Mysteries, the secret ceremonies that would one day draw initiates from every corner of the Greek and Roman world. She taught him the things that could not be spoken, the sights that could not be described, the knowledge that transformed the fear of death into the hope of a blessed afterlife. Celeus became the first mortal to receive this gift, and from him it passed to his son Triptolemus, to the priest Eumolpus, and to the generations of hierophants who would preside over the Mysteries for nearly two thousand years. The Homeric Hymn to Demeter, the oldest and most authoritative account of these events, names Celeus among the leaders to whom Demeter showed her sacred rites, alongside Triptolemus, Diocles, Eumolpus, and Polyxeinus.

The later traditions added darker details to Celeus' story. Apollodorus, writing in the first or second century CE, claimed that Celeus was killed in a war, though he did not specify which war or against whom. Hyginus said that Celeus was one of the men who tried to assault Demeter when she was disguised, and that she punished him with death, though this contradicts the older and more widely accepted tradition of his hospitality. Pausanias, the Greek traveller who visited Eleusis in the second century CE, recorded that the Eleusinians preserved a threshing floor and an altar of Triptolemus at the sanctuary, and he noted that the locals still pointed out the well where the daughters of Celeus had found the goddess. The well itself was shown to visitors, a modest stone structure that had once changed the course of Western religion.