The territory was occupied from the late Bronze Age and used by Elymians, Romans and Byzantines for its command of the western approaches to Palermo; Roman kilns survive in contrada Foggia, datable between the first and fourth centuries AD.

The town itself is Arab. Founded as a casale -station on the commercial road between two defensive outposts, it appears in the historical record in 1154 , in the geographer Edrisi's Book of Roger , which calls it a mazil and describes fertile land and a market. A generation later, in 1185 , the diary of an Andalusian pilgrim who stopped here calls it Beleda — a town with mosques and a market. The etymology of the name is disputed: some read alquam , muddy ground; others Manzil Alqamah .

Under the Normans and then Frederick II of Swabia , after a series of Saracen revolts, the Muslim population was deported and the casale gradually Christianised. What followed was the making of the town as it stands. In 1340 the Peralta began building the castle; it was completed around 1350 by the brothers Enrico and Federico Chiaramonte , then lords of Alcamo. Together with the castles of Salemi and Calatafimi it formed a fortified defensive system. Soon after came the mother church, the market square and the Franciscan convent, and Alcamo became a significant centre of the island.

The Castello dei Conti di Modica takes its name from the family that held it longest — over four centuries. It has a rhomboidal plan with four towers, two square and two cylindrical alternating at the corners, wall-walks with pointed arches, and two- and three-light windows. The emperor Charles V stayed here in 1535 . When feudal power ended in 1812 , the building passed on; in 1828 the comune took possession and used it as offices, a prison and a stable, which is why its interior arrangement has been altered so many times and why the outer walls are gone.

Between the fourteenth and seventeenth centuries a succession of religious orders settled here and the town became an important commercial centre, filling with the palaces and churches that still define it — including the Sant'Oliva of Antonello Gagini (1511).

And Alcamo has a literary claim of the first order. The thirteenth-century poet Cielo d'Alcamo — Ciullo — was born here, author of the thirty-two-stanza contrasto beginning Rosa fresca aulentissima , one of the founding texts of Italian vernacular literature.