Argus was Hera's servant. His great service to the Olympian pantheon was to slay the chthonic serpent-legged monster Echidna as she slept in her cave. Hera's defining task for Argus was to guard the white heifer Io from Zeus, who was attracted to her, keeping her chained to the sacred olive tree at the Argive Heraion. She charged him to "Tether this cow safely to an olive-tree at Nemea". Hera knew that the heifer was in reality Io, one of the many nymphs Zeus was coupling with to establish a new order. To free Io, Zeus had Argus slain by Hermes. The messenger of the Olympian gods, disguised as a shepherd, first put all of Argus' eyes asleep with spoken charms, then slew him by hitting him with a stone, the first stain of bloodshed among the new generation of gods. After beheading Argus, Hermes acquired the epithet Argeiphontes or “Argus-slayer”.

The sacrifice of Argus liberated Io and allowed her to wander the earth, although tormented by a gadfly sent by Hera, until she reached the Ionian Sea, named after her, from where she swam to Egypt and gave birth to a love child of Zeus, according to some versions of the myth.

According to Ovid, to commemorate her faithful watchman, Hera had the hundred eyes of Argus preserved forever, in a peacock's tail.