Zeus was born to the Titan Cronus and his sister-wife Rhea, the last of six children. Cronus, having overthrown his own father Uranus, had been warned by Gaia and Uranus that one of his own children would one day overthrow him in turn. To prevent this prophecy, Cronus swallowed each child as soon as Rhea gave birth: first Hestia, then Demeter, Hera, Hades, and Poseidon. When Rhea became pregnant with Zeus, she sought help from her parents Gaia and Uranus. Following their plan, she travelled to Lyctus in Crete and gave birth to Zeus in a cave on Mount Ida or Mount Aegaeon. She left the infant with Gaia to raise, nursed by the goat Amalthea and guarded by the Curetes, young warriors who clashed their weapons to drown out the baby's cries so Cronus would not hear him. Rhea then returned to Cronus and presented him with a stone wrapped in swaddling clothes, which he swallowed without examining it. This stone, the Omphalos, would later be placed at Pytho on Mount Parnassus as a sacred relic.

When Zeus grew to manhood, he returned to confront his father. According to some accounts, the Oceanid Metis gave Zeus a potion or emetic to mix into Cronus's drink. When Cronus swallowed it, he was forced to vomit up all the children he had devoured, now fully grown: Hestia, Demeter, Hera, Hades, and Poseidon. The stone came up last. Zeus now had his siblings at his side, but he needed more allies to challenge the Titans. He descended into the deepest abyss of Tartarus and freed the Cyclopes, three one-eyed giants who were master craftsmen, and the Hecatoncheires, three monstrous beings with a hundred hands and fifty heads each. In gratitude, the Cyclopes forged weapons for the young gods: a thunderbolt for Zeus, a trident for Poseidon, and a helmet of darkness for Hades that made the wearer invisible. The Hecatoncheires, with their overwhelming strength and ability to hurl massive boulders, swelled the ranks of Zeus's army.

The war that followed was called the Titanomachy, the war between the old Titans and the young Olympian gods, and it lasted ten years. The Titans, led by Cronus and with Atlas as their battlefield commander, fought from Mount Othrys while Zeus and his allies made their base on Mount Olympus. The earth shook and the heavens thundered as the two sides clashed. Several Titans, including Themis and her son Prometheus, sided with Zeus. Eventually the Olympians and their allies overpowered the Titans. Cronus and most of his supporters were cast into the dark pit of Tartarus, imprisoned behind bronze gates. Atlas was given a unique punishment: he was condemned to stand at the western edge of the world and hold up the heavens upon his shoulders for eternity. Zeus, Poseidon, and Hades then drew lots to divide the cosmos: Poseidon received the sea, Hades the underworld, and Zeus the sky and the heavens, though the earth remained common to all three.

But Zeus's reign was not yet secure. Gaia, angered by the imprisonment of her children the Titans, bore a final monstrous son: Typhon, a creature with a hundred serpent heads that breathed fire, whose body stretched across the horizon and whose voices roared like bulls, lions, and hissing serpents all at once. Struck with terror, the other Olympian gods fled to Egypt, transforming themselves into animals to hide. Zeus alone stood his ground. He faced Typhon in a battle that shook the earth to its foundations. Armed with his thunderbolts, Zeus struck the monster again and again until Typhon was cast down into Tartarus beneath the volcanic Mount Etna, where his struggles are said to cause the eruptions that still thunder from that peak. This victory cemented Zeus's position as the undisputed king of gods and the protector of cosmic order.

Zeus's rule was marked by both justice and caprice. He married his sister Hera, making her queen of Olympus, though his many affairs with goddesses and mortal women produced a vast divine and heroic lineage. From his union with the Titaness Metis, Athena sprang fully grown and armed from his forehead after he swallowed Metis to prevent a prophecy that her child would overthrow him. With Leto he fathered the twins Apollo and Artemis. With Maia he fathered Hermes, the messenger god. With Semele, a mortal woman, he fathered Dionysus, the god of wine and ecstasy. With Alcmene he fathered Heracles, the hero forced to complete twelve impossible labours. He presided over the assembly of the gods on Olympus, settled disputes, enforced oaths, and sent thunderbolts against those who broke divine law. He was worshipped across the Greek world at Dodona, where priests interpreted his will from the rustling of oak leaves, and at Olympia, where the Olympic Games were held in his honour every four years. His image in the great temple at Olympia, a colossal seated statue of ivory and gold created by the sculptor Phidias, was one of the Seven Wonders of the Ancient World.