The very ancient origins of the town, originally called Fundi, are lost in myth; legend says that the site was founded by Hercules in memory of the killing of Caco, a character from Roman mythology linked to fire.

During the pre-Roman era, the area was inhabited by the Aurunci and later by the Volsci. Although there are no solid historical sources, it seems that the town surrendered to the Romans and in 388 B.C. it was granted the right of limited citizenship and, only in 188, full citizenship.

Located along the route of the Via Appia, Fondi benefited from the commercial and economic advantages linked to its favourable position. Subsequent invasions by the Longobards first of all and then by the Saracens, seriously damaged the prosperity of the town.

It was only thanks to Pope John VIII that the area was freed of invaders, with the naval battle of the Circeo in 877. The town was theatre to the famous adventure of the attempted abduction of Giulia Gonzaga, whose fame and prodigious beauty attracted Barbarossa, who, in 1534 attempted to abduct the noblewoman so that he could give her to the sultan Suleiman.

However the women managed to escape and the famous predator took his anger out on Fondi, setting fire to it. In 1594 numerous other incursions by Turkish pirates tested the stability of the town greatly and in 1720 Fondi started to decline for ever, as a result also of the malaria that was developing in the marshy areas of the plain and the growing spread of brigantry during the French occupation.

The ancient settlement kept its original rectangular layout centred on the Via Appia, divided by decumans and cornerstones according to the classical layout of the Roman camp. The location of the ancient Forum has been identified near the duomo.

Within the medieval town, in via S. Angelo, there is the house where Michele Pezza, otherwise known as Fra Diavolo, was born, and it is also possible to visit the medieval castle.