Peleus was born on Aegina, the island in the Saronic Gulf that his father Aeacus had made famous as a kingdom of justice and order. Aeacus was the son of Zeus and Aegina, the nymph from whom the island took its name, and he had three sons: Peleus, Telamon, and Phocus. Phocus was the favourite, beautiful and athletic, the son of a different mother, and his brothers grew jealous of the attention he received. The jealousy turned to murder. In some accounts, Peleus and Telamon plotted together to kill Phocus during a discus throw, hurling the stone disc at his head. In others, Peleus acted alone, striking his half-brother in a moment of rage. Whatever the truth, Phocus died, and Aeacus, in his grief and his sense of justice, exiled both Peleus and Telamon from Aegina forever. Peleus wandered northward to Thessaly, where he was purified of his blood-guilt by Eurytion, the king of Phthia, and given the hand of Eurytion's daughter Antigone in marriage, along with a third of the kingdom. He became a king in exile, a man who had murdered his way out of one life and built another from the wreckage.

In Thessaly, Peleus proved himself a warrior of the first rank. He sailed with Jason and the Argonauts, the crew of the ship Argo who sought the Golden Fleece in the distant land of Colchis on the Black Sea, and he fought in the great hunt of the Calydonian Boar, the monstrous creature sent by Artemis, the goddess of the hunt, to ravage the fields of Aetolia. In that hunt, he accidentally killed his father-in-law Eurytion with a javelin cast amiss, a second death by his hand that forced him to flee Phthia and wander again. He found refuge on Mount Pelion, in the cave of Cheiron, the wisest of the Centaurs, the creature who was half-man and half-horse and who trained the greatest heroes of the age. Cheiron took Peleus in, taught him the arts of war and medicine and music, and restored him to a kingship that he had twice lost through blood.

The defining episode of Peleus's life was his courtship of Thetis, the sea-nymph whose beauty was so great that both Zeus and Poseidon had desired her. Thetis was one of the Nereids, the fifty daughters of Nereus, the Old Man of the Sea, and she had been raised by Hera, the queen of Olympus, who loved her as a foster daughter. When Zeus and Poseidon learned of a prophecy that Thetis would bear a son greater than his father, they withdrew their suits, for no god wished to suffer the fate that had befallen Uranus, castrated by Cronus, and Cronus, overthrown by Zeus. Thetis must marry a mortal, the gods decided, and Peleus, with his divine grandfather and his proven courage, was chosen as the bridegroom. But Thetis did not submit willingly. She was a goddess, or nearly so, and she would test the mortal who claimed her.

The test was a wrestling match of supernatural ferocity. Peleus found Thetis sleeping in a cave on the coast of Thessaly, and he seized her and held on as she transformed herself into every shape the natural world could produce. She became fire, and her body burned with a heat that should have reduced a mortal to ash, but Peleus held on. She became water, and her body flowed like a river through his fingers, but Peleus held on. She became a lion, and her claws tore at his flesh, but Peleus held on. She became a serpent, and her fangs struck at his throat, but Peleus held on. She became a tree, and her bark hardened into iron, but Peleus held on. Through every transformation, his grip never loosened, and finally Thetis, exhausted and impressed, returned to her true form and consented to be his wife. The wedding was celebrated on Mount Pelion, and all the gods attended. Apollo played the lyre, the Muses sang, and the Centaurs danced, and from the union of Peleus and Thetis was born Achilles, the hero whose name would outlast every other name of the age.

Peleus's later life was shadowed by loss. Thetis, unable to bear the mortality of her husband, left him and returned to the sea, though she continued to watch over her son. Peleus raised Achilles alone, with the help of Cheiron, and he gave the boy the divine armour and the immortal horses that would carry him to Troy. When the Trojan War began, Peleus was already an old man, too old to fight, and he watched his son sail away with the fleet that Agamemnon had gathered at Aulis. He knew the prophecy: Achilles would die young and famous at Troy, and he would not see his father again. Peleus outlived his son by several years, ruling Phthia in lonely old age, and he died before Neoptolemus, Achilles's son by the Trojan princess Deidamia, returned from the war with the ashes of his father. The tomb of Peleus stood in Phthia, and the Greeks who passed through Thessaly on their way to Delphi or Olympus would stop to honour the king who had held a goddess in his arms and who had paid for that glory with the loss of everything he loved.