The archaeological findings indicate that the settlement in Ios dates back to the Early Cycladic period and because of its geographical location it had a significant growth during the 3rd millennium. It is known that during the period of the classical era it belonged to the Delian League and it paid initially one talent as a tribute to the Athenians. According to the indications that there are Ios did not participate in the alliance against the Athens known as «Common of the Islanders». It seems, however, that it participated when it was re-established in the Ptolemaic period. A commemorative plaque in honour of the wife of Ptolemy II Philadelphus, Arsinoe reveals a possibility that Arsinoe was the name of the island at that time.
During the Roman occupation, Ios was part of the «Province of the Islands» (Provincia Insularum) and exiles were sent to island. It was occupied in 1207 by Marco Sanudo and it was annexed to the Duchy of Naxos. In 1269 it returned to the hands of the Byzantine and in 1292 it was reconquered by Domenico Schiavi with the assistance of the galleys Mark II Sanudo. The dominance of Schiavi lasted until 1320, when the island was given as compensation to Marco II Sanudo.
The Duchy of Naxos remained in power until 1397, when after the death of Francesco Crispo, it was given to Mark Crispi together with Therasia Island. Because of repeated pirate raids the population had diminished and then Crispo favoured the settlementof Arvanites colonists from the Peloponnese. At the same time, he built the castle of Chora. In 1450 he was succeeded by his eldest son, Jacopo, who was childless and as a result the fief of the island went to his uncle Francesco II and later to his son Marco III. He gave the island as a dowry to his daughter Adriana who married in 1508 the Venetian nobleman Gian- Luigi, or Aloisio Pisani. The house of the Pisani remained in Ios until 1556 when the island permanently passed to the Ottomans. In 1537 it was conquered by Barbarossa who imposed tribute. During the Rule of the Turks the island was known under the name Ansa or Aine.
Ios in 1558 suffered the consequences of the raid of 14 Turkish pirates and in 1579 it was re-colonized by 200 Arvanites who arrived here under the leadership of a priest, Pothitos, with the support of Beylerbey (senior military officer) of the islands. Historical sources indicate that during the Ottoman period itwas part of the kaza (province) of Athens and taxation was leased from 1550 and for three years to Mehmet Turali for the sum of 80.000 akçes.
During the Russian -Turkish War (1768-1774) Ios was occupied by the Russians and was ruled by three local Syndiki (governors). After the Treaty of Küçük Kaynarca (1774) and until 1821 it returned to the Turkish dominance.
Panagiotis Amoirakis raised the flag of the Revolution on the island and asked weapons to be sent to the island with a letter he wrote on the 16th May 1821. As an important personality of the post-revolutionary Greece Spyridon I. Vallettas emerged, originally a member of the Society of Friends (FilikiEteria), later Minister of Education and a great benefactor of the island.