Kasos is mentioned by Homer in the Iliad, as participating in the Trojan War with ships. Traces of Mycenaean settlements have been found in the southern part of the island. However, the residents abandoned it to transfer to the more protected and fertile north. The new settlement developed around today's Poli, as indicated by the archaeological findings from the area. Only few things are known about Kasos' history, such as the fact that in 1537 it was transfered into Ottoman administration. During the Ottoman occupation, Kasos enjoed commercial privileges and prospered economically.

Just before the 1821 Revolution, Kasos has approximately 12.000 residents and a 100 sheep fleet. With these forces it participates actively in the uprising, succeeding in giving decisive strikes to the Turkish fleet. However, the activity of the people of Kasos constitutes the island a target. So, in May 1824, the Turkish-Egyptian fleet attacks the island but the inhabitants resist strongly, managing to repel the enemy; although unfortunately, only temporarily. On June 7, several thousands of soldiers land on the island in Antiperato, at a point where a traitor from Kasos suggested. Residents resist and fall fighting heroically, only for the merciless slaughter to follow. It is estimated that about 7000 were killed or sold as slaves in Egypt. For several years the island remained deserted, and when people started to gradually return, it never regained its population and economic power.

In 1911, Kasos is under Italian administration, until it permanently joins Greece in 1948 together with the rest Dodecanese.