The Roman bridge that crossed the Garonne at a place called Apas, near St. Martory, indicates that the territory was occupied in Roman times. In the early 12th century., The region was owned by Abbey Lézat (Ariege) and then became one of the abbey of Bonnefont from its inception. In the 16th century., Castillon and the nearby village of Saint-Médard-en-Comminges, both ruled by the nearby abbey, form a single entity. In 1741, Castillon became just as Saint-Médard a separate entity. The village name comes from the Latin word meaning small castle castellionem.