Lynceus was born in Messenia, the fertile region of the southwestern Peloponnese, the son of Aphareus, the king who ruled from the city of Arene beside the river Pamisus, and his wife Arene. His brother was Idas, a man of such violent temper and such reckless courage that the two brothers were known throughout Greece as a pair who should never be crossed. Lynceus was the quieter of the two, a man whose power lay not in his muscles but in his eyes, which the gods had endowed with a sight beyond any mortal. He could see through solid rock, through the bark of trees, through the darkness of a moonless night. He could count the leaves on a distant oak, spot a fish beneath the surface of a turbulent sea, and track a fox across a landscape of bare stone where no footprint could remain. The Greeks said that his eyes were a gift from the gods, or a curse, or simply the natural perfection of a man born to a line of kings who had always been closer to the divine than ordinary mortals.

When Jason assembled the crew of the Argo, the ship that would sail to Colchis in search of the Golden Fleece, Lynceus and Idas were among the first to volunteer. They came from Messenia with their father's blessing, and Lynceus's eyes proved invaluable on the voyage. When the Argonauts were becalmed in the dark, he could see the currents beneath the surface that told them where to row. When they landed on unknown shores, he could scan the hills for enemies before the others had even stepped onto the sand. When the Harpies, the winged spirits of sudden death, attacked the blind prophet Phineus, it was Lynceus who spotted them first, crying out their approach while they were still distant specks against the sky. The other Argonauts respected him, but they also feared him, for a man who can see everything is a man who knows secrets that others would prefer to keep hidden.

After the voyage of the Argo, Lynceus and Idas returned to Messenia and married. Idas married Marpessa, the daughter of Euenus, the river-god, after carrying her off in his chariot and defeating Apollo himself in a contest for her hand. Lynceus married Hypermnestra, one of the fifty Danaids, the daughters of Danaus who had fled from Egypt to Argos to escape forced marriage to their cousins, the fifty sons of Aegyptus. On their wedding night, Danaus had given each of his daughters a dagger and commanded them to kill their husbands, but Hypermnestra alone refused, sparing Lynceus because he had treated her with kindness. When her sisters were punished in the underworld for their crime, condemned to fill leaky jars with water for eternity, Hypermnestra was spared, and she and Lynceus became the ancestors of the royal house of Argos, the line that would one day produce Perseus, the slayer of Medusa.

The defining episode of Lynceus's life was the feud with the Dioscuri, the twin brothers Castor and Polydeuces. The quarrel began over cattle, as so many Greek feuds did. Idas and Lynceus had driven their herd to a certain pasture, and the Dioscuri had driven theirs to the same place, and both pairs claimed the land by ancestral right. Rather than fight immediately, they agreed to settle the matter by a contest: one man from each side would eat an entire cow, and the side whose champion finished first would win the pasture. Idas, who was a prodigious eater, finished his cow before Polydeuces had eaten half of his, and the brothers drove the Dioscuri's cattle away in triumph. But Castor and Polydeuces were not men to accept defeat. They ambushed Idas and Lynceus as they were dividing the spoils, and a battle broke out that would leave three men dead.

In the fighting, Lynceus used his divine sight to spot Castor hiding inside a hollow oak tree, or beneath the earth in a tomb, depending on the version. He saw what no other man could see, and he drove his spear through the hiding place and killed the mortal twin. Polydeuces, who was immortal because his father was Zeus, went mad with grief. He pursued Lynceus across the plain, and though Lynceus could see him coming, he could not outrun the son of a god. Polydeuces caught him and killed him with a single blow of his fist, or with a spear, or with the thunderbolt that Zeus lent him in that moment of divine rage. Idas tried to avenge his brother, but he too was killed, struck down by Zeus himself with a thunderbolt for daring to attack an immortal. The feud ended with three corpses on the Messenian plain and Polydeuces weeping over the body of Castor, begging Zeus to let him share his immortality with his dead brother.