During the Roman Empire, the Ionian Islands (Corfu, Cephalonia, Zakynthos, Ithaca, Leucas and Cythera) were variously part of the provinces of Achaea and Epirus vetus.

[3][4][5] The islands remained largely unaffected by the Slavic invasion and settlement of the 7th century, and formed a base for the re-establishment of imperial control and the re-Hellenization of the mainland coast.

A number of seals further push the establishment of the circumscription of Cephallenia, at least as a strategis if not as a theme, back to the middle or late 8th century.

In 880, the admiral Nasar heavily defeated an Arab pirate fleet that was plundering the theme's islands, and troops from Cephallenia subsequently participated in the Byzantine offensive in Italy.

[19] Following the collapse of Byzantine control in southern Italy in the mid-11th century, the theme's importance declined, and it was headed by civilian governors, styled krites ("judges").

Although Corfu was recovered by the Byzantines by 1191, the other islands henceforth remained lost to Byzantium, and formed a County palatine of Cephalonia and Zakynthos under William's Greek admiral Margaritus of Brindisi.