Endymion was born in Thessaly, the mountainous region of northern Greece, the son of Aethlius and Calyce. His father Aethlius was the son of Zeus and Protogeneia, the daughter of Deucalion, the man who survived the great flood that destroyed the world, and his mother Calyce was the daughter of Aeolus, the king who gave his name to the winds. In some accounts, particularly those from Elis, Endymion is said to be the son of Zeus himself, born to a woman named Phoenissa. He grew to be a man of such extraordinary beauty that no mortal could match him, and he proved himself a leader as well as a beauty. He led the Aeolian people from Thessaly westward across Greece and founded the city of Elis, in the fertile plain of the western Peloponnese, where he established himself as king. The people of Elis would later claim him as their founding hero, and they showed his tomb at Olympia beneath the very racecourse where the Olympic Games were held.

The defining moment of his life came when Selene, the Titan goddess of the moon, daughter of Hyperion and Theia, saw him sleeping in a cave on Mount Latmus in Caria, a mountain in the region of Asia Minor near the city of Miletus. The poet Sappho, writing in the sixth century BCE on the island of Lesbos, tells us that Selene visited him every night in his cave, and the Roman poet Ovid, in his Heroides, describes the moon pausing in her course across the sky to gaze down upon the sleeping shepherd. Selene fell so deeply in love with Endymion's beauty that she could not bear the thought of his aging and death. She went to Zeus, the king of the gods, and begged him to grant her beloved eternal youth. Zeus, who could not refuse the moon herself, offered Endymion a choice of destinies, and Endymion chose to sleep forever, remaining deathless and unaging in an eternal slumber. In some versions, recorded by the scholiast on Apollonius of Rhodes, Zeus allowed Endymion to be the keeper of his own death, to decide when he would die, and Endymion chose never to wake. In a darker tradition, recorded by the poet Theocritus, Zeus punished Endymion with eternal sleep because the mortal had dared to fall in love with Hera, the queen of the gods, a crime that other mortals such as Ixion had paid for with far worse torments.

While Endymion slept in his cave, Selene came to him every night. The poet Lucian, in his satirical Dialogues of the Gods, imagines Selene confessing to Aphrodite, the goddess of love, that she glides down from her moon chariot and steals kisses from the sleeping shepherd, treading on tiptoe so as not to wake him. By him, according to the geographer Pausanias, she bore fifty daughters called the Menai, the goddesses of the lunar months, each one representing a month of the four-year Olympiad cycle. Some scholars, including James George Frazer, have interpreted this as a mythological explanation for the calendar: Endymion is the sunken sun, overtaken by the moon below the horizon, and his fifty daughters are the fifty lunar months. The Roman naturalist Pliny the Elder, writing in the first century CE, offered a rationalising explanation: Endymion was not a lover but the first astronomer, the first man to observe and record the movements of the moon, and the nightly "visits" were simply his observations.

Before his sleep, Endymion had already established a dynasty. He married Asterodia, or in some accounts Chromia or Hyperippe, and fathered several children who would shape the history of Greece. His sons were Aetolus, who slew Apis and fled to the Curetian country, naming it Aetolia after himself; Epeius, who won a race at Olympia for the throne of Elis and gave his name to the Epeans; and Paeon, who went into exile beyond the river Axius and gave his name to Paeonia. He also had a daughter named Eurycyda. The people of Elis held a race at Olympia to determine which of Endymion's sons would inherit the kingdom, a tradition that may lie behind the founding of the Olympic Games. When Endymion finally entered his eternal sleep, the Eleans buried him at Olympia beneath the racecourse, while the people of Heracleia near Miletus claimed he had retired to Mount Latmus and built him a shrine there. Pausanias, who visited both sites in the second century CE, recorded both traditions and noted that he had seen a statue of Endymion in the treasury of the Metapontines at Olympia.

Endymion's image endured long after the fall of Greece. The proverb "Endymionis somnum dormire," to sleep the sleep of Endymion, entered Roman literature through Cicero's Tusculanae Quaestiones and the poetry of Propertius. In Late Antiquity, when the afterlife became a pressing concern, scenes of Endymion and Selene were among the most popular subjects for sculpted sarcophagi: the image of a beautiful youth sleeping eternally while a divine figure watched over him was the perfect allegory for death that was not death, a peaceful rest that preserved beauty beyond the grave. The Roman poet Ovid told the story in his Heroides and Ars Amatoria, and in the Renaissance the myth was transferred to Diana, the Roman equivalent of Artemis, though the virgin goddess was an awkward fit for a tale of divine passion. In 1818 the English poet John Keats published his four-thousand-line poem Endymion, with its famous opening line "A thing of beauty is a joy for ever," turning the sleeping shepherd into a symbol of the eternal pursuit of beauty itself. From the cave on Mount Latmus to the Louvre, from the racecourse at Olympia to the pages of Romantic poetry, Endymion remains the mortal who chose sleep over waking and won immortality in the arms of the moon.