Castelnaudary is mentioned in a document from 1118 under the name ‘Castellum Novum Arri’ ie the new castle of Arius. The town was built around this castle (now the Lauragais Museum). Castelnaudary became a strategic stronghold during the crusade against the Cathares. In 1211, it fell into the hands of the Croisés, then in 1221 it was taken over by the Comte de Toulouse.
In 1355, in the midst of the Hundred Years War, the town was burnt down by the Black Prince, son of King Edward III of England. The Lauragais region and its capital was not at peace until the reign of Henri II of France. His wife, Catherine de Medici, became countess of Lauragais and made Castelnaudary the seat of a Sénéchaussée, an administrative and judicial district. She built the Présidial, a civil and criminal court, that attracted many lawyers and judges from Toulouse.
At the beginning of the 17th century, the town witnessed an episode during the Fronde. In 1632, a terrible battle took place below the town. The Duke of Montmorency was captured and beheaded at the Capitol. The 17th century also saw the creation of the Canal Royal du Languedoc, now called the Canal du Midi .
Pierre Paul Riquet, its brilliant inventor, made Castelnaudary an important port by developing the (7 ha) Grand Bassin there. During the Revolution, Castelnaudary became sub-prefecture of Aude, a title that it lost in 1926 to Carcassonne. Since 1976, the town has hosted the Foreign Legion and remains the capital of Lauragais and Cassoulet.