Way back in time Kassos was first inhabited by Phoenicians who named it "The island of sea foam".

Later it was settled by Dorians previous to the Trojan War (1193 - 1184BC) which was recorded by Homer in the Iliad, and he mentions the fact that Kassos too joined in the campaign against Troy.

Kassos has retained the same name since Homer's time, previously having been called successively Amphi, Astravi and Achni.

During the Persian Wars (490 - 479BC) all the islands of the Eastern Mediterranean, including Kassos, were dominated by the all-powerful Athenian State (as recorder in 4379C) and had to contribute 1000 drachmas annually.

However, according to Demosthenes the general in his Rhodian War the unity of the islands was broken, and they eventually submitted to Macedonian rule. Throughout Alexander the Great's campaign Kassos maintained a neutral stance. During the wars with King Mithridates V (87 - 65BC), all the islands put up a stout resistance to this ruler of the Barbarians on the Euxine Pontus.

The Roman civil war following the assassination of Julius Caesar in 53BC found the islands on the decline, but still retaining their autonomy until the reign of Empero  Vespasian (69 - 79AD). Thereafter Rhodes and the rest of the islands were decisively subjugated by the Romans, and finally, in the reign of Diocletian (284 - 305AD) they formed part of the 18th Province of the Roman Empire, Kassos being linked with Cre¬te. Later on, in 730AD the islands of the Eastern Aegean were named Dodecanisa (from "dodeka" - twelve and "nisia" - islands).
During the period 825 - 961 AD the Spanish-based Arabs made their appearance, capturing and occupying Crete from which they made their onslaughts on the islands. The Emperor Nikephorus Phokas of Byzantium successfully put an end to them, rid¬ding the Aegean of a scourge, soon however to be replaced by Venetians, Genoese and Franks.

At all times all the islands, Kassos included, were harassed by pirates – Melitians, Turks, Tunisians, Algerians, Tripolitans, Doultsineans, Cretans, Maniots and KephalIonians.
In 1537 Kassos and Karpathos were occupied  by the Turks. During the Turkish oc¬cupation all the islands of the Dodecanese enjoyed a privilege granted by Sultan Suleiman, according to which all Turkish generals, admirals and civil officials, should they at any time come into contact with the islanders, were forbidden to maltreat them or to interfere in their affairs. For this reason they paid a special tax in kind twice annually. They were allowed self-government by elected elders of the community. The only Turkish official present was the so-called Soumbasis who refrained from interfering in regional issues.

According to accounts of Greek and foreign travellers, Kassos was deserted and in ruins from 1579 -1599. Settling in before 1622, she detached herself from the Arch¬bishopric of Karpathos and established herself as a Patriarchal Province. In 1670 the population totaled 5,000.

During the Orlof Revolution (1768 - 1774) the island was occupied by the Russians who repeatedly embroiled themselves in clashes with the Greeks. After the signing of the Koutjouk Kainartji treaty (10 -12 July 1774), the Russians evacuated the islands and they were once more under Turkish domination.