The first inhabitants were Achaeans. Kassos participated in the Trojan War, according to Homer. Later the island was colonized by Dorians. Kassos was part of Byzantine empire and suffered a lot by Arab pirates and Latin crusaders, until it was invaded by turks.

In the 18th century, the island developed into a significant naval power. The islanders participated in the Greek Revolution of 1821, led by Captain Bourekas. In June 1824 Ibrahem pasha of Egypt sent Hussein bey with 4000 Albanians, who burned down the island and enslaved all women and children. Men over 8 years old were massacred. In 1912 the island was conquered by Italians and in 1948 finally was liberated and became part of Greece.