The presence of Etruscan, Roman and Lombard place names indicates that the territory has been inhabited since antiquity, although the towns of today are mentioned in written records only starting from the 10th and 11th centuries. The first mention of "Castelvecchio di Cascia" appeared in a document from 1066.
Castelnuovo di Cascia, situated in the vicinity of the Romanesque Parish Church of San Pietro, developed instead during the 12th century along the Cassia Vetus, an ancient Roman road that linked Florence with Arezzo along a route that partially coincides with the Via dei Setteponti at Reggello.
Cascia was under the influence of the Florentine Republic, which from 1309 on had organized its territory into "leagues" ruled by a Podestà.
The League of Cascia governed a territory as large as that of the present municipality, to which were added the populations of the League of Incisa in 1424.The existence of the town of Reggello has been documented since 1250. Today’s municipal capital probably originated as a marketplace at the crossroads between a byway of the Via Cassia and the Casentino road that, across the mountain, connected the Valdarno to the Casentino Valley. In the late Middle Ages, the town of Reggello became so important that, in the early 15th century, both the seat of the Podestà and the market were moved there.
However, the League and the Podestà continued to bear the name of Cascia until 1773, when the Community of Reggello was established through a legislative provision of Grand Duke Leopoldo. The Sant'Ellero-Saltino railway was inaugurated in September 1892, promoting the development of tourism in the hill and mountain area.