The Phoenicians were, most probably, the first residents of Posidonia. Some archaeological evidence in the area of Halepa indicates the existence of an early civilization by this name. In the surrounding area of Agios Petros church (which is built over the temple of god Poseidon) one can still see the ruins of an ancient settlement. In fact, the islet of Schinonissi, opposite to Agathopa beach has been populated as early as 2800-1500 BC.
Later in history, the area served mainly for farming and pasture, since the place was constantly threatened by corsair attacks and consequently its population moved to safer, fortified settlements like Ano Syros. Around 1823 and on, the corsair raids had ceased to threaten the area, so Posidonia was repopulated, mostly by refugees from other suffering parts of Greece during the times of the Greek War of Independence. The main activities of this population at that time had to do with farming, cheese-making, viniculture and fishing.
The architectural image of Posidonia, with its simple, well-kept stone houses, has a different color in comparison to the rest of the islands in Cyclades. On the other hand, the village has many sumptuous villas which were built in the area back when many wealthy families chose Posidonia as their summertime resort. The rest of the area has well designed houses which are populated by numerous families. On the exterior of those houses you can see a feature called senazi (from the french chaînage). It is a strip of stone or concrete on the upper part of the exterior wall, painted with light blue or pink colors to contrast the white walls.