Before the Roman conquests of Gaul, the territory of Arlon and a vast area to the southeast were settled by the Treveri, a Celtic tribe. The local population adapted relatively easily to Roman culture. The number and quality of sculpted stones and monuments that have been unearthed in the area demonstrate that the vicus of Orolaunum quickly became a commercial and administrative centre of Roman civilization. The Germanic invasions of the 3rd century destroyed most of these early advances, despite the defensive walls that had been built on the Knipchen hill to protect the vicus.

During most of the Middle Ages, the population still used the earlier buildings such as the thermae. In 1060, Waleran I of Limburg, Count of Arlon, built a castle on the Knipchen hill. In the 13th century, the only women's Cistercian abbey known to date was built in Clairefontaine.

Arlon is best known for holding one of the richest archeological museums in Belgium. It houses numerous examples of Roman sculpture and Merovingian funerary art.

A fragment of the Gallo-Roman defensive wall that was built in the 3rd century still stands in Arlon.

The Gaspar Museum is dedicated to the art of Jean-Marie Gaspar and Charles Gaspar, and also holds a collection of regional art including the Fisenne altarpiece, an altarpiece from the 16th century originally located in the village Fisenne.

Saint Donat's church now stands on the Knipchen hill, where Waleran I of Limburg once built his castle and the Capuchin friars built their convent.

Arlon cemetery has the largest Jewish section of all Walloon cemeteries.