The territory of Valentano has been inhabited since prehistoric times; there are important finds of places dwelling villages in the Lake Mezzano.
There is little evidence about the Etruscan era, restricted to the areas of Spinetto, Vallone, Saint Lucia and Monte Becco. Instead there are many artifacts from the Roman era, such as roads and rustic villas.
In the Middle Ages, the first news of the city in its present name dates back to a manual of 813 P.C. Belonging to the Summary of Farfa, while the "Balentanu" moniker appears in other documents related to the Abbey of San Salvatore on Mount Amiata from 844 P.C.
Linked to this period we also have the presence of the ruined castle of Mezzano (827 P.C.) and the villages of Villa delle Fontane (839 P.C.).
In the middle of the fourteenth century the town became a possession of the Farnese, whose indelible traces are found in Valentano itself and in the surrounding countries. The Farnese settled permanently in Valentano, after Cardinal Albornoz gave them the fortress built by the Farnese themselves along with numerous churches.
The various disputes between the Farnese and the Apostolic Campaign ended up with the War of Castro (1649 P.C.).
With the destruction of Castro, Valentano became the administrative center of the Castrense and here was transferred the historical archive.
After this series of events, around the first half of the eighteenth century, the Farnese left the village and the fortress was transformed into a monastery until 1930.
The city was part of the League of Municipalities of Castro which opposed to the temporal power of the Popes. It was the seat of a garrison of Papal Zouaves and while celebrating the capture of Rome (1870), the Zouaves set fire to the historical archive of the City cards, so there are no more tracks left.
During the two world wars the fallen were many, both military and civilian, and to them was dedicated in 2004 a commemorative plaque, placed in the are where, during the Second World War, a shot of artillery caused the death of seven civilians.