The first signs of settlement in this region have been dated to about 3500 BC, but the hill site of Carsac—a Celtic place-name that has been retained at other sites in the south—became an important trading place in the sixth century BC.

The folk etymology—involving a châtelaine named Lady Carcas, a ruse ending a siege, and the joyous ringing of bells ("Carcas sona").

A medieval fiefdom, the county of Carcassonne, controlled the city and its environs. It was often united with the county of Razès. The origins of Carcassonne as a county probably lie in local representatives of the Visigoths, but the first count known by name is Bello of the time of Charlemagne. 

Carcassonne became famous for its role in the Albigensian Crusades when the city was a stronghold of Occitan Cathars. In August 1209 the crusading army of the Papal Legate, abbot Arnaud Amalric, forced its citizens to surrender. Viscount Raymond-Roger de Trencavel was imprisoned while negotiating his city's surrender and died in mysterious circumstances three months later in his dungeon. The people of Carcassonne were allowed to leave—in effect, expelled from their city with nothing more than the shirts on their backs. Simon de Montfort was appointed the new viscount and added to the fortifications.

In 1240, Trencavel's son tried unsuccessfully to reconquer his old domain. The city submitted to the rule of the kingdom of France in 1247. Carcassonne became a border fortress between France and the Crown of Aragon under the 1258 Treaty of Corbeil. King Louis IX founded the new part of the town across the river. He and his successor Philip III built the outer ramparts. 

In 1659, the Treaty of the Pyrenees transferred the border province of Roussillon to France, and Carcassonne's military significance was reduced. Its fortifications were abandoned and the city became mainly an economic center of the woollen textile industry, for which a 1723 source quoted by Fernand Braudel found it "the manufacturing center of Languedoc"