The first time the name appears is in the Sigillum Aureum of 1086, the act by which Roger of Hauteville founded the diocese of Mileto: among the villeins he gives to the bishop there is «the priest Peter of Saint Kónos». Some scholars translated that name as San Cono — but in the eleventh century no saint of that name yet existed: the two Conos venerated by the Church lived in the following century, and the Byzantine tradition knew several Conones but no Cono. The name written in the parchment, then, is almost certainly Costantino. The village's name was recovered by proving that the saint it had been attributed to had not yet been born. The first settlement was probably Byzantine, on a slope hard to reach, sheltered from the wind and close to springs; then the Normans gathered the people scattered through the countryside into a walled citadel, Francica, and San Costantino became one of its hamlets.
Two earthquakes damaged the village in the seventeenth century, and the one of 1783 razed it: nine people died out of the five hundred and eight who lived here, and part of the settlement was rebuilt higher up, on level ground and to a regular plan. That position — away from the marshy areas where malaria raged and close to the old Roman road — made it grow: between the eighteenth and the nineteenth century many families moved here from Pongadi, a village in the Mesima valley a few kilometres off, which was abandoned altogether precisely because of malaria, and in a few years San Costantino had more inhabitants than Francica, the comune it was a hamlet of. Napoleon's government gave it autonomy in 1807, the reorganisation of 1811 took it away, and it became a comune in earnest only in 1854. Meanwhile the village had begun a church far beyond its means, with a dome and two bell towers, so slow in the building that after eighty years some parts were still without plaster: the earthquake of the morning of the eighth of September 1905 destroyed it. The new church, smaller, was opened for worship on the eighth of September 1930, the same day of the year.