Archaeological findings reveal that Achaea has been inhabited since ancient times. The first to inhabit the region were the Pelasgians, who called it Aigialeia. Later the Ionians settled in and named the area Ionia, when in 1088 BC it was Achaeans' turn to settle and gave it its present name, Achaea. The Achaeans founded the city of Patras by joining existing settlements on the initiative of Patreas, from whom it is said to have gotten its name.

During Roman times, Patras flourished as a point of strategic importance. In the early 13th century Achaea is occupied by the Franks who divide it into 48 small fiefdoms. 28 of them constituted the barony of Patras, 12 the barony of Kalavryta and 9 the barony of Aigio. In 1430 Constantine Palaiologos (Palaeologus) occupies the Castle of Patras and releases Achaea. However, the joy this brings does not last long as Achaea is forced to follow the fate of the rest of the country and falls in 1460 into the hands of the Turks. They maintain it until the Greek Revolution of 1821, with a slight respite between 1687 and 1715 that Achaea comes into the Venetians' possession. Patras and the wider region of Achaea lead the revolution, being one of the first regions to shake off the Turkish yoke and regain freedom. Unfortunately though, it seems that Achaea was meant to pay another big toll, at this time during the Second World War. The German army destroyed Kalavryta and exterminated the male population, leaving over 800 dead in what was called the Massacre of Kalavryta.