Phrixus was born in Orchomenus, the great city of Boeotia in central Greece, the son of King Athamas and Queen Nephele. His mother was a cloud-nymph, a minor goddess of the mist and the rain whom Zeus had given to Athamas as a sign of divine favour, and his birth was celebrated as a blessing upon the kingdom. He had a sister named Helle, younger and more delicate, and for a time the family was happy. But Athamas grew restless in his marriage to a creature of mist, and he cast Nephele aside to marry Ino, the daughter of Cadmus, the founder of Thebes, and Harmonia, the daughter of Ares and Aphrodite. Ino was beautiful, ambitious, and ruthless, and she bore Athamas two sons of his own: Learchus and Melicertes. The new queen saw Phrixus and Helle not as stepchildren but as rivals, obstacles to her own sons' inheritance, and she resolved to destroy them.

Her method was cunning and terrible. She persuaded the women of Orchomenus to roast the seed corn before it was planted, so that when the sowing season came, nothing grew from the earth. The crops failed, the cattle sickened, and famine spread across Boeotia. Athamas, desperate to save his people, sent messengers to Delphi, the oracle of Apollo, to learn what sin had brought this disaster. Ino intercepted the messengers and bribed them to return with a false answer: the famine would not end until Phrixus and Helle were sacrificed to Zeus upon the altar of Mount Laphystius. Athamas, a king who loved his children but loved his kingdom more, believed the lie and prepared to carry out the sacrifice.

Phrixus and Helle were led to the mountain, dressed in the white garments of victims, and the priests prepared the altar and the knife. But Nephele, watching from her place in the sky, saw what was happening and would not let her children die. She sent a golden ram, a creature with a fleece of pure gold and the power of flight, down from the heavens. The ram appeared beside the altar, and the children, understanding that their mother had sent it, climbed upon its back. The ram leaped into the air and carried them across the sky, its golden fleece blazing in the sunlight. But Helle, the younger and more frightened, grew dizzy with the height and the speed. She loosened her grip, slipped, and fell into the narrow strait that separates Europe from Asia. She drowned before her brother's eyes, and the strait was named the Hellespont, the sea of Helle, forever after.

Phrixus, weeping, clung on. The ram carried him across the Aegean Sea, over the mountains of Asia Minor, and across the vast expanse of the Black Sea until it reached Colchis, the wealthy kingdom on the eastern shore ruled by King Aeetes, the son of Helios, the sun-god. Aeetes received the boy with hospitality, for the Greeks believed that a man who arrived on a divine beast must be favoured by the gods. He gave Phrixus his daughter Chalciope in marriage, and the union produced four sons: Argus, Melas, Cytissorus, and Phrontis. These sons would one day sail with the Argonauts, guiding Jason back to their grandfather's kingdom to claim the treasure their father had left behind.

The golden ram had served its purpose, and Phrixus knew that it must be returned to the gods. He sacrificed the ram to Zeus, the king of the gods, and he gave its magnificent fleece to King Aeetes as a gift of gratitude. Aeetes hung the fleece upon an oak tree in a sacred grove dedicated to Ares, the god of war, and set a dragon to guard it, a sleepless serpent whose coils wrapped around the tree and whose eyes never closed. The fleece remained there for a generation, its golden glow visible across the kingdom, until Pelias, the king of Iolcus, sent his nephew Jason to fetch it, launching the voyage of the Argo and the most famous adventure in Greek mythology. Phrixus lived out his days in Colchis, honoured as a prince and a survivor, and when he died he was buried with respect, though some accounts say that Aeetes eventually killed him in a fit of suspicion or that he died of old age in a foreign land, never seeing his father's kingdom again.