Kea has been inhabited from ancient times. The prehistoric settlement of Kefala which can be found on the peninsula with the same name on the northwest coast of the island, started during the late Neolithic period, around the end of the 4th millennia (3300 BC approximately). The prehistoric settlement of Saint Irene Keas, on the north side of the port of Saint Nikolas (main port), was one of the most important cultural centers of the Aegean area from the end of the Neolithic period (3000 BC) when the first settlement was established in the area, until the 15th century BC, when it was destroyed at its peak by strong earthquakes.

During the archaic times (7th-6th century BC) there were four independent, economically and culturally strong city states on Kea: Ioulis (currently the main village), Karthea, Poiessa and Korissos (currently the port). Ruins of these cities can be found today showing the development and importance of the island during antiquity. The Archeological Museum in the main village, hosts a large collection of findings from all over the island. A walk to ancient Karthea is one of the most memorable walking trips you can have on the island.