Its origin is unknown, but the first certain news dates back to 852 A.D. where, in a bull by Pope Leo IV addressed to the bishop of Tuscania, the name Montis Alti appears for the first time.
Between the 11th and 12th centuries, Montalto suffered severe destruction. The best known is that of 1109 when Pope Paschal II, in order to defeat Stephen of the Corsicans, had Castrum Montis Alti, where he had castled himself, destroyed by the Norman militia.
During the Middle Ages, a port of call arose and developed: located at the mouth of the Fiora River, it was a very important port of call for the Tyrrhenian routes, especially for the grain boat.
The historical centre reached its maximum urban expansion in the 14th century and became a territory disputed by the Pope, the Municipality of Rome, the Orsini family and the Prefects of Vico.
The constant battles of this era, the Avignon Captivity and the difficult economic situation put the village in serious crisis; the difficult condition would improve, in part, in 1421 when Pope Martin V, interested in maintaining a settlement in the area to guard the Dogana dei Pascoli, wrote a bull to encourage repopulation in the area.
From this moment on, the territory of Montalto became inextricably linked to the agro-pastoral system, transhumance and seasonal work.