Cadmus was credited by Herodotus with introducing the original Phoenician alphabet to the Greeks, who adapted it into their own script; modern scholarship has largely agreed with him on the Phoenician source of the alphabet, even while dating the historical transfer much later than the mythical Bronze Age setting of Cadmus's Theban foundation. Herodotus claimed to have seen Cadmean writing on tripods in Apollo's temple at Thebes and estimated Cadmus sixteen hundred years before his own time.

In early accounts Cadmus and Europa are sometimes children of Phoenix rather than of Agenor. Theban tellings could modify his maternity (for example naming a daughter of the Nile-god Nilus). His descendants ruled Thebes on and off for several generations, including into the age of the Trojan War.

After Zeus carried Europa from the Phoenician shore, Cadmus was ordered by his father not to return without her. Unsuccessful — or unwilling to oppose Zeus — he came to Samothrace, sacred to the Great Gods (Kabeiroi). Traditions there pair him with his mother Telephassa and with Thasus, and Diodorus aligns the company with Electra and her sons Dardanus and Iasion, whose sister Harmonia Cadmus took as bride. Their wedding was said to be the first on earth at which the gods brought gifts and dined with the couple.

At Delphi the oracle told him to abandon the search and follow a cow marked with a half-moon on her flank, founding a city where she lay down exhausted. The cow, given by Pelagon of Phocis, led him to Boeotia. Intending to sacrifice her to Athena, Cadmus sent companions for water to the Ismenian spring; the spring's guardian dragon slew them, and Cadmus killed the dragon in turn. Athena then told him to sow the dragon's teeth, from which sprang the Spartoi ("sown men"). By casting a stone among them he set them fighting until five survivors remained — Echion among them — who helped build the Cadmeia and founded Thebes's noblest houses.

Because the dragon was sacred to Ares, Cadmus served the god eight years in penance. At the end of that service the gods gave him Harmonia as wife. Their children include Polydorus, Illyrius, and the daughters Agave, Autonoë, Ino, and Semele. Bridal gifts included a peplos worked by Athena and a necklace made by Hephaestus — the Necklace of Harmonia — which brought misfortune to later possessors. Despite the divine wedding, the house of Cadmus was overtaken by grief; he eventually abdicated in favor of his grandson Pentheus and went with Harmonia to Illyria, fighting for the Enchelii and founding cities such as Lychnidos and Bouthoe.

Troubled by the curse that clung to him for killing Ares' dragon, Cadmus once remarked that if the gods so loved a serpent's life he might wish that life for himself — and immediately began to grow scales. Harmonia begged to share his fate; the gods granted her request. In another telling their bodies changed after death while their souls went to the fields of the blessed. In Euripides's Bacchae, Dionysus prophesies that both will become snakes for a time before dwelling among the blessed.

Modern scholarship has long debated Cadmus's eastern origin. Homer already calls Thebans "Cadmeans"; Euripides is among the earliest tragedians to stress "Cadmus the Tyrian," while Herodotus popularizes the Phoenician prince. Hypotheses have cast him as a fictitious eponym of the Cadmeia made Phoenician by eastern contacts, as a reflection of Near Eastern dragon-combat stories, or as a Mycenaean or Argive figure reframed; a proposed reading of a Hittite letter naming a historical Cadmus among the Ahhiyawa is rejected by most scholars. The Syrian city of Al-Qadmus bears his name, and the chemical element cadmium is named after him.