In a series of documents and records of scholars, as well as texts from foreign authors, we find evidence that the history of Longos can be traced back to earlier times.
In fact, as early as Venetian and Franconian times, we find that the region of Longos was inhabited by free farmers.This is stated in a document published by Ernesto Gerland in 1395 and in various other documents on the town of Patras from the years 1397 and 1440 respectively. Refering to the "Short Index of Villages of the Pelopponese or Morea" we learn that the village of Longos had already existed in 1629. In Grimanis Venetian Index of 1770 it is listed as "Longo" a name probably referring to what was commonly known as a region rich in scrubland and forests.
Furthermore, in 1805, the French diplomat, historian and traveller, Pouqueville, made a note of the village of Longos and its modest chapel. The village of Longos is also mentioned by Sir William Gill, in his book "Travelogues of Morea" (1829) while on his way from Vostizza (Egio) to Patras.
In the years following the liberation, Longos clerically belonged to the diocese of Patras. In a document which refers to the years 1830 to 1832 the Cypriot priest, Agathangelos Myrianthousis, speaks of the priest Ioannis and the monk Benediktos. In the year 1830 the village of Longos with its 20 families was registered for the first time in the province of Aigialia, in a register that was started during an expedition by the French general, Maizon. The Ottonian Archives of the Interior Ministry (1835) intergrate Longos in the town of Egio (Aigieon) of the province Aigialias and Helidos which is nowadays the prefecture Ilias.
In his book "Greek Villages" (1903) the scholarly doctor, Christ. Koryllos, describes Longos as a large village with 388 inhabitants and having its own administration. It belonged to the town of Egio until 1912 when in the same year the municipality of Longos was founded.
In 1977 this municipality was again dissolved and was made part of the County of Sympolitia and from Januar 1, 2011 Longos belongs to the new founded County of Aigialeiou.