Thetis was born to Nereus, the old man of the sea, a primordial sea god of great wisdom and truthfulness who lived in a cave beneath the waves, and Doris, the daughter of Oceanus, the great river that encircled the world. She was one of fifty daughters, the Nereids, the sea nymphs who accompanied Poseidon and calmed the waves with their dancing. Thetis was the most beautiful of them all, and her grace was such that Zeus himself, who had overthrown his father Kronos and made himself king of the gods, fell in love with her. Poseidon, his brother and the ruler of the oceans, also desired her, and for a time the two gods competed for her favour. But Themis, the Titan goddess of divine law, or in some versions the prophet Prometheus, who had stolen fire for mankind and was punished by being chained to a rock, delivered a prophecy that changed everything: the son of Thetis would be greater than his father. Zeus and Poseidon, both of whom had overthrown their own fathers, understood the danger immediately. Neither dared to marry Thetis, for the prophecy meant that any son she bore to a god would one day overthrow that god, just as Zeus had overthrown Kronos and Kronos had overthrown Ouranos. Thetis was instead given in marriage to a mortal, Peleus, the son of Aeacus and the brother of Telamon, a hero of great strength but limited lifespan, so that the son who surpassed his father would still be mortal and no threat to Olympus.

Peleus won Thetis through a trial of endurance and cunning. He found her on the shore of Thessaly, dancing with her sisters in the surf, and seized her in his arms. Thetis, enraged, transformed herself into every shape she could command: she became a lion, roaring and clawing; she became a serpent, coiling and striking; she became fire, blazing and burning; she became water, flowing and slipping through his fingers. But Peleus held on, his grip never loosening, his arms wrapped around her through every transformation, until finally Thetis, exhausted and impressed, resumed her true form and consented to be his wife. The wedding was held on Mount Pelion in Thessaly, the mountain where the centaur Chiron, the wise creature who was half man and half horse, trained the greatest heroes of Greece. Every god and goddess attended except Eris, the goddess of discord, who had not been invited. In revenge, Eris threw a golden apple inscribed with the words "To the fairest" into the midst of the wedding guests, and the quarrel over who deserved it, between Hera, Athena, and Aphrodite, would eventually lead to the judgment of Paris and the Trojan War that would kill Thetis's only son.

Thetis bore Peleus a son, Achilles, and from the moment of his birth she knew that he was destined for both greatness and sorrow. She tried to make him immortal. In the most famous version of the story, she took the infant Achilles to the River Styx, the sacred water that flowed around the Underworld and whose touch granted invulnerability. She held him by the heel and dipped his body into the dark water, and every part that the water touched became impervious to wound or blade. But the heel by which she held him remained dry, and that single spot of mortality would one day be pierced by the arrow of Paris, guided by Apollo, the god of archery, and would bring about Achilles's death. In another version, told by Stesichorus, Thetis tried to burn away Achilles's mortality by placing him in the fire each night and anointing him with ambrosia, the food of the gods, but Peleus discovered her and stopped her, and the boy remained mortal. In either telling, Thetis's love for her son was absolute, and her grief at his fate was the driving force of her divine life.

When the Trojan War began, Thetis knew that Achilles was fated to die young if he went to Troy, for a prophecy had declared that he would either live a long and obscure life at home or die young and glorious at Troy. Achilles chose glory, and Thetis let him go, but she watched over him from the sea. When Agamemnon, the king of Mycenae and commander of the Greek forces, dishonoured Achilles by taking his war-prize, the slave girl Briseis, Achilles withdrew from the fighting in rage and begged his mother to intervene. Thetis rose from the sea at night, her silver feet barely touching the sand, and went to Olympus to beg Zeus to let the Trojans drive the Greeks back to their ships, so that Agamemnon would learn the cost of insulting her son. Zeus, bound by the promise he had made to her at a previous supplication, agreed, and for days the Trojans, led by Hector, the greatest of the Trojan warriors and the son of King Priam, battered the Greek camp, killing hundreds and threatening to burn the ships.

The crisis deepened when Patroclus, Achilles's closest companion, borrowed Achilles's armour and led the Myrmidons into battle, only to be killed by Hector. Achilles's grief was so terrible that it overcame his rage at Agamemnon, and he resolved to rejoin the war and kill Hector, even though he knew that his own death would follow soon after Hector's. But he had no armour, for Hector had stripped Patroclus's corpse and taken the divine armour that Thetis herself had given her son. Thetis heard Achilles's lament from the depths of the sea and rose again to Olympus, this time to beg Hephaestus, the lame smith-god who forged weapons for the Olympians, to make a new suit of armour for her son. Hephaestus, who owed Thetis a debt of gratitude for having once sheltered him when Hera had cast him from Olympus in disgust at his deformity, worked through the night at his forge beneath the volcano, hammering bronze and gold into a shield of extraordinary beauty, a breastplate that shone like fire, greaves of tin, and a helmet with a golden crest that blazed like a star. The shield was decorated with scenes of human life: cities at peace and at war, ploughing and harvest, dancing and justice, the whole world rendered in metal, a vision of what Achilles was fighting to protect and what he would never live to see.

Thetis brought the armour to Achilles at dawn, weeping as she laid it before him, knowing that it would carry him to his death. Achilles accepted it and went forth to kill Hector, dragging the Trojan prince's body behind his chariot in his fury. Thetis did not intervene again until Achilles himself lay dead, pierced by the arrow of Paris that struck the one vulnerable spot on his body, the heel she had held so long ago. She rose from the sea with her sisters, the Nereids, and they mourned Achilles for seventeen days, their keening echoing across the waters. Thetis herself held her son's body and anointed it with ambrosia, preserving it from decay, and she promised that his fame would live forever, a promise that Homer, singing centuries later, would keep. She also intervened in the quarrel over Achilles's armour, which was contested between Odysseus and Ajax the Great after Achilles's death, though in the end it went to Odysseus by trickery, and Ajax, driven mad by the shame, fell upon his own sword.