Proteus was born to Oceanus, the Titan whose river encircles the entire world, and Tethys, his sister and wife, the goddess of the fertile waters. He was one of the Old Gods of the Sea, a deity of the generation before Poseidon, the Olympian god of the earthquake and the grey sea, and he retained his ancient power even after the younger gods took their thrones on Olympus. While Poseidon ruled the broad waters with his trident, Proteus tended the flocks of seals that basked upon the rocks of Pharos, the island that lies at the mouth of the Nile in Egypt. Each day at noon he would rise from the waves and count his seals, lying down among them on the warm stones, an old man with a flowing beard and skin the colour of the sea-foam.

The defining episode of Proteus's mythic career came during the wanderings of Menelaus, the king of Sparta whose wife Helen had been taken to Troy and whose fleet was now scattered across the Mediterranean on the long voyage home. Menelaus and his ships were becalmed on the island of Pharos, trapped by contrary winds for twenty days while his men grew desperate and his stores ran low. The sea-nymph Eidothea, one of Proteus's daughters, took pity on Menelaus and told him the secret of wresting prophecy from her father. She advised him to take three of his strongest men and hide among the seals at noon, when Proteus emerged to count his flock. Then they must seize the old god and hold him fast, no matter what forms he took.

Menelaus followed her counsel. He and his men smeared their bodies with seal-skin to mask their scent, lay down among the basking seals, and waited. When Proteus emerged from the waves and lay down to sleep, they sprang upon him. The god thrashed and changed shape with terrifying speed, becoming first a lion with a mane of flame, then a serpent coiling and hissing, then a panther, then a boar, then running water, then a towering tree. But Menelaus held on with the strength of a drowning man clinging to wreckage, and when Proteus saw that he could not break free, he resumed his true form and spoke. He told Menelaus that he must first make proper sacrifice to the gods, then revealed that his brother Agamemnon, the king of Mycenae who had commanded the Greek forces at Troy, had been murdered by his wife Clytemnestra and her lover Aegisthus upon his return to Mycenae. He told Menelaus that he must not sail directly home but must travel up the Nile to Egypt and perform the rites that would appease the gods and grant him a safe passage. And he told him what every hero most fears and most desires to know: the manner of his own death, that Menelaus would not die in battle or by treachery but would be carried to the Elysian Fields, the paradise of the blessed dead, because he was the husband of Helen and thus a son-in-law of Zeus.

In another tale, the shepherd-god Aristaeus, the son of Apollo, the god of music and prophecy, and the nymph Cyrene, came to Proteus in despair. His bees had died, every hive empty and every swarm vanished, and he did not know what sin he had committed to bring such ruin upon his livelihood. His mother Cyrene sent him to Proteus, and again the old god was seized and held until he spoke truth. Proteus told Aristaeus that he had neglected to honour the nymphs who guarded his bees, and that he must sacrifice four bulls and four heifers, leave their carcasses in a grove for nine days, then return to find that bees had swarmed from the rotting flesh. Aristaeus obeyed, and the bees returned. This was the power of Proteus: he did not give his wisdom freely, but once forced to speak, his words were never false.

Proteus was also the father of Eidothea, the sea-nymph who helped Menelaus, and of the seal-herds themselves, which were sacred to him. In later ages, his name became a byword for changeability and versatility. The word protean entered the English language to describe anything that shifts form or character with ease. His island of Pharos became the site of the Lighthouse of Alexandria, one of the Seven Wonders of the Ancient World, built by Ptolemy II in the 3rd century BCE and standing for fifteen centuries before earthquakes brought it down. The lighthouse bore his name because it stood upon his island, and so the name of the shape-shifting sea-god became the word for any guiding beacon in a dark place.