Orneus was the son of King Erectheus of Athens. He was the brother of Metion. When Erectheus died, his brother Cecrops II became king. However, Metion soon overthrew Cecrops and took power for himself. After Metion died, the kingship passed to Cecrops' son Pandion II, who was overthrown by some of Metion's sons. However, Aegeus, Lycus and Nisos returned their father (Pandion II) to power in Athens, expelling Metion's descendants.
After the death of Pandion, Aegeus succeeded as king and, in turn, was succeeded by his son Theseus. When Theseus died, the kingship passed to Orneus' grandson (through his son Peteus), Menestheus; the last of Erectheus' descendants to rule Athens.