Augusta Auscorum was born of the fusion of an Aquitain-based people living on the right bank of the Gers and the Roman culture that was imposed here after the conquest by Julius Caesar. The fall of the Roman empire brought the city and its entire region under Visigoth domination. Later, the Merovingian and Carolingian dynasties defined its institutions throughout the medieval era, as the region struggled through insecurity and hardships of all kinds. Among the early invaders were the Vascones who lent their name to the former Roman province in the 6th century: Vasconia would evolve into Gascony, of which Auch became the archiepiscopal centre.
Major invasions would influence the migration of the population toward Roman hill forts, and the city began to take on its characteristic appearance as a medieval town, of which many elements still remain. During this era, the town was flooded with pilgrims on their way to Santiago de Compostela.
Auch did not undergo any noteworthy changes until the 18th century, during which the intendant D'Etigny would transform and beautify it. Urban planning projects carried out under the Second Empire would confer it its modern appearance.
One of the Auch's identities today is as city of modern circus arts. The Centre de Recherche et d'Innovation Circacienne organises the Festival du Cirque Actuel (Festival of Modern Circus Arts) in Auch every year at the end of October.