Aethlius was born from the union of Zeus, the king of the gods, and Protogeneia, the daughter of Deucalion and Pyrrha, the only mortals who survived the great Flood that Zeus sent to cleanse the earth of its wickedness. Deucalion had built an ark and floated for nine days while the waters covered every mountain peak, and when the flood receded he and Pyrrha cast stones behind them that became the new race of men. Protogeneia was one of their children, and through her Aethlius carried the blood of both the thunder god and the survivors of the deluge. Some later traditions, seeking to connect the kings of Elis to the great Aeolian houses of Thessaly, called him a son of Aeolus, the wind-king, but the older and more widely accepted genealogy makes him a direct descendant of Zeus through the flood line.

When Aethlius came of age, he led a band of Aeolians from Thessaly southward into the northwestern Peloponnese, where the land was rich with rivers and olive groves. There he founded the kingdom of Elis, a territory that would one day include the sacred grove of Olympia, where Zeus himself was worshipped above all other gods. Aethlius took Calyce as his wife, a princess of the wind-king's house, and she bore him at least one son, Endymion, a youth of such surpassing beauty that poets would sing of him for a thousand years. Some say Endymion was himself a son of Zeus, born to Calyce while Aethlius raised him as his own, a common pattern in Greek myth where the king's wife is visited by the god and the mortal husband gives the child his name and his kingdom.

The most distinctive act of Aethlius' reign, in the later chronicle of Eusebius, was the setting of athletic contests. Eusebius has him draw upon the ancient tradition of the Idaean Dactyls, the finger-shaped spirits of Mount Ida in Crete who were said to have invented racing and wrestling, and challenge his sons to compete against one another in trials of speed and strength. His name, Aethlius, shares its root with Greek athlos, a contest for a prize, and with athletes, one who competes for that prize. The name and the games belong to the same word-family; the English word athlete does not descend from the king himself. After Aethlius died, his sons Epeius and then Endymion, along with Alexinus and Oenomaus, continued to preside over the sacrifices connected with these early games. The tradition passed from father to son until it blossomed into the Olympic Games that would draw competitors from every corner of the Greek world.

Aethlius' greatest legacy, however, was not the games but the son who outshone him. Endymion grew to be the most beautiful mortal ever born, a shepherd-king whose face was so perfect that Selene, the Titan goddess of the moon, fell in love with him at first sight. Every night she descended from the sky to visit him in a cave on Mount Latmos, and when she saw that mortal beauty must fade with age, she begged Zeus to grant Endymion a gift. Zeus offered him anything he desired, and Endymion chose to sleep forever, remaining deathless and ageless in eternal slumber, visited each night by the moon. The Eleans showed a tomb for Endymion, but the people of Heracleia near Miletus claimed he slept still on Mount Latmos, honoured in a shrine that drew pilgrims from across the ancient world. Through his son, Aethlius' bloodline became intertwined with the heavens themselves.

When Endymion came to rule Elis, he set his own sons to run a race at Olympia for the throne. Epeius won, and the people were first called Epeans after him. The dynasty of Aethlius continued through Endymion's children, including Aetolus, who gave his name to Aetolia, and Eurycyda, who married Poseidon and bore Eleius, the king from whom the Eleans took their name. The kingdom that Aethlius founded with a band of Thessalian wanderers became one of the most sacred territories in Greece, the guardian of Olympia and the Olympic truce. Aethlius himself faded from the stories, eclipsed by his immortal son and his athletic descendants, but every runner who ever sprinted down the stadion at Olympia, every crown of wild olive placed upon a victor's brow, traced its origin back to the prize-winning king.