The first document attesting the existence of Sonnino is a papal bull of the year 999.

In those times the village was already inhabited and it is difficult not to link its foundation to the process of fortification underway in the Church State. This process involved the creation of a long series of fortified settlements on high that were at the same time shelters for the population, threatened by wars and incursions, and military outposts. The fortified village was ruled by a chivalrous family of Lombard origin, the Sompnino, less known of the Pagano di Ceprano or the riotous Ceccano and Aquino. The domains of Sompnino are documented by sources from the twelfth and thirteenth centuries, as in 1248-49 when Jordan and James fought a violent "war" against Veroli. However, at the beginning of the thirteenth century even the Frangipanes of Rome considered themselves lords of Sonnino and at an unknown moment during the thirteenth century the castle passed under the direct authority of the Apostolic See.

On 11 October 1369 half of the castle was bought for 2000 florins by Onorato I Caetani, who also became lord. Subsequently the castle was inhabited until 1496 by the Caetani of Aragon, under the papacy of Alexander VI became the possession of Rodrigo Borgia and the death of the pope (1503), the feud passed to the Colonna family, who maintained the domain almost continuously until 1816, year of their renunciation of feudal rights in the territory.

By a bull of 13 January 1596, Pope Clement VIII established the Principality of Sonnino. Other lords who lived there for short periods were the Borgia and the Carafa. The last owners were the Antonelli and the Talani. The ancient municipal statute, dating from the thirteenth century, is preserved in the State Archives of Rome. With the Restoration and the reform of 1816 Sonnino returned to the baronial place belonging to the Colonna, but already in 1817 it became podesteria within the delegation of Frosinone, district government of Terracina. From 1800 to 1825 the Sonninese territory is characterized by the presence of bands of brigands.

The history and myth of the brigand leader Antonio Gasbarrone, a native of Sonnino, represent a characteristic of an area that has expressed a population celebrated in the chronicles and European iconography for the rebellious nature of its inhabitants.

In the 1827 territorial division Sonnino appears as podesteria dependent on Piperno, while in that of 1831 it is a community subject to an extraordinary commissioner within the district of Pontecorvo. It remained until 1870 in the Papal States within the province of Campagna and Marittima, on the border with the Kingdom of the two Sicilies. After the annexation to the Kingdom of Italy, which occurred with the plebiscite of 2 October 1870, the Municipality became part of the province of Rome and then, in 1934, the newly established province of Latina.

Currently, with a population of 7,585 inhabitants, the town has a remarkable historical center surrounded by about 500,000 olive trees. Pride of the village, in fact, is the production of oil and olives of Gaeta. His landscapes in the area of the "Neapolitan papal confinement of 1840" are of immeasurable beauty. The landscape views are joined by historical finds that testify to the thousand-year history of the "Terre di Confine".