Cadmus was born in Tyre, the great Phoenician city on the coast of what is now Lebanon, the son of King Agenor and Queen Telephassa. He had several brothers, including Cilix and Phoenix, and one sister, Europa, whose beauty was so extraordinary that Zeus himself desired her. The king of the gods came to Tyre in the guise of a gentle white bull, mingled with the royal herds, and when Europa climbed upon his back he sprang into the sea and swam to Crete, where he revealed his true form. King Agenor was devastated and enraged. He sent his sons forth with a terrible command: find Europa, and do not return without her. Cadmus set out with his brothers, each taking a different direction, and searched the Mediterranean for years. But Europa was beyond finding, hidden on Crete and destined to become the mother of the kings of that island, including Minos, the legendary lawgiver whose name would one day be given to the civilisation of the Minoans.

When Cadmus had exhausted every lead, he came to Delphi, the most famous oracle in Greece, where a priestess called the Pythia sat above a chasm and spoke prophecies inspired by the god Apollo. The oracle did not tell him where Europa was. Instead it gave him a new command: stop searching for his sister. Follow a cow that bore the mark of the moon upon its flank, and build a city on the spot where the beast lay down to rest. Cadmus obeyed. A cow with a half-moon mark appeared, and he followed it across Phocis and into Boeotia, the rich plain north of the Gulf of Corinth. At last the cow stopped beside a spring called the Ismenian Spring, near a place that would one day be called Thebes. Cadmus was overjoyed. He wished to sacrifice the cow to Athena, the goddess of wisdom and war, and sent some of his companions, men named Deioleon and Seriphus among them, to draw water from the spring for the ritual.

The spring was guarded. A monstrous serpent, the offspring of Ares in some accounts, a creature with a golden crest and triple rows of teeth, lay coiled in the cave from which the water flowed. When Cadmus's men dipped their pails into the pool, the dragon emerged and killed them all, crushing some in its coils and poisoning others with its venom. Cadmus waited for their return, and when they did not come he followed their tracks to the spring and found their bodies. He attacked the serpent with stones and then with his spear, and after a fierce battle he pinned the creature against an oak tree and killed it. A mysterious voice spoke from the air, warning him that he too would one day become a serpent and be stared at as he had stared at the dragon, but Cadmus had no time to ponder the prophecy. Athena appeared to him in the aftermath and told him to take the dragon's teeth and sow them in the earth like seeds.

Cadmus did as the goddess commanded. He tore the teeth from the dragon's jaw and scattered them across the ploughed field. From the furrows, armed men began to spring up, fully grown, wearing helmets and carrying spears and shields. They were the Spartoi, the sown men, born from the earth itself. At first they simply stood and stared at one another, but then one of them cried out that a jewel had been thrown among them, and they turned on each other in a frenzy of suspicion and violence. The battle was terrible, earth-born warriors hacking at earth-born warriors until the ground ran red. Only five survived: Echion, Udaeus, Chthonius, Hyperenor, and Pelorus. These five became Cadmus's allies and the founding ancestors of the noble families of Thebes. Together they built the citadel called the Cadmeia, and around it rose the city that would become one of the greatest in Greece, the stage for the tragedies of Oedipus, Antigone, and the house of Labdacus.

But the dragon had been sacred to Ares, and the god of war demanded a price. Cadmus was forced to serve Ares for eight years as penance for the killing. When the term was complete, the gods themselves arranged his marriage to Harmonia, the daughter of Ares and Aphrodite, the goddess of love. Their wedding was the first occasion on which gods and mortals feasted together as equals. Twelve golden thrones were set for the Olympians, and the Muses sang. Athena gave the bride a robe woven by herself, and Aphrodite gave her a golden girdle. But Hephaestus, the divine smith and husband of Aphrodite, was bitterly jealous of his wife's affair with Ares, and he crafted a cursed necklace as a wedding gift for Harmonia, a jewel that would bring misfortune to every woman who wore it.

Cadmus and Harmonia had four daughters and two sons. Their daughters were Semele, who would become the mother of Dionysus, the god of wine and ecstasy, only to be burned alive by Zeus's thunderbolt when she asked to see his true form; Ino, who would be driven mad by Dionysus and leap into the sea with her infant son; Autonoe, whose son Actaeon was torn apart by his own hounds for glimpsing the goddess Artemis bathing; and Agave, who in a Dionysiac frenzy would tear her own son Pentheus limb from limb upon the mountains of Cithaeron. Their sons were Polydorus, who died young, and Illyrius, from whom the Illyrians claimed descent. The curse of the necklace worked its way through the generations, and the house of Cadmus became synonymous with suffering.

In his old age, Cadmus could bear Thebes no longer. He and Harmonia left the city they had built and wandered north to the land of the Enchelei in Illyria, on the shores of the Adriatic. There, according to Euripides in his tragedy The Bacchae, Cadmus was transformed into a serpent by Dionysus himself, punished for denying the god's divinity despite being his own maternal grandfather. In other accounts, the transformation was voluntary or sent by the gods as a release from human sorrow. Harmonia, who loved her husband beyond measure, prayed to share his fate and was transformed into a serpent beside him. Together they slithered through the Illyrian forests, still capable of human speech and thought, until at last the gods took pity and sent them to the Elysian Fields, the green meadows of the underworld where the blessed dead dwell in peace beneath a sunless sky.