According to some studies the name Arnara derives from the Lombard Arnhari, Latinized into Arnarius. Others say the name comes from “Arnaro” meaning “closed place” as a cave or a hollow in the earth.
According to Suetonius, the Roman settlement was founded when Caesar distributed the land among his 20,000 veterans. Through here passes the Via Casilina, the ancient Via Latina, and it was identified as a staging post for changing horses.
The actual urban centre dates back to the arrival of the Lombards. Arnara formed around the tower built by the Lombard king Astolfo in 756, then transformed into a castle, on a hill over the river Sacco. The purpose of the castle was to counter the papal troops.
The first information on the Village of Arnara date back to 1121 in the Chronicle of the Abbey of Fossanova where this fiefdom is in the county of Ceccano. The castle has undergone numerous attacks. From 1466 Arnara returned to be among the properties of the Colonna, which were the owners until the early twentieth century.
During World War II the town was severely damaged by the violence of war.