Leonessa was founded in 1278, by merging various castles that already stood on the plateau at the foot of Monte Tilia. That plateau was a frontier: from the end of the twelfth century the boundary line between the Papal State and the Kingdom of Sicily ran along its northern edge, and each of the two had its own group of castles on that side. The town is the result of putting some of them together. Its two surviving gates still say which way it looked: the Porta Spoletina faces north, towards Spoleto, and the Porta Aquilana east, towards L'Aquila — and it was to L'Aquila that Leonessa belonged, in Abruzzo, for more than six centuries. It changed region only in 1927, passing to the newly created province of Rieti. In the fifteen hundreds it was enfeoffed to Margherita of Austria, daughter of Charles the fifth, and it was she who in 1548 had the octagonal fountain built that stands in the main square, with its coats of arms, its dolphins and its masks.
After the eighth of September the country around Leonessa filled with partisans, and in March 1944 the town and its hamlets were occupied by the Garibaldi brigade and entered a free zone that ran from the Valnerina to Norcia and Leonessa, with Cascia at its centre. The German army attacked in force, and between the second and the seventh of April fifty-one civilians were killed here, among them the parish priest, don Concezio Chiaretti. The main square of the town bears the date of the last of those days. Leonessa also keeps the list of its earthquakes, and it is a long one: 1298, twenty years after the foundation, with the epicentre close by; more in the fourteenth and fifteenth centuries; then the great earthquake of 1703, which flattened villages in this territory and killed eight hundred people; and finally the Valnerina in 1979, Colfiorito in 1997 and Amatrice in 2016 — these last three without a single death.