Füssen was settled in Roman times under the name "Fauces" at the Via Claudia Augusta, a road connecting Northern Italy with Augusta Vindelicum (= Augsburg), the former regional capital of the Roman province Raetia. The Via Claudia Augusta developed into an important trade route over the Alps.

In 748 the missionary St. Magnus set up a "cella" on-site here. More monks arrived and in the 8th century the Benedictine monastery of St. Mang, named after him, existed. Füssen was an important place Southern and Northern Europe, as the Lech river was navigable near Füssen and the cargo was reloaded here from horses to ships. The route gained importance as a salt road in the Middle Ages.

Füssen was owned by the House of Staufer from 1191 on, but after the last Staufer Conradin (aka "Conradin the Kid) was executed in Naples in 1268, the ownership got disputed.

In 1313, Emperor Henry VII gave it to the bishopric of Augsburg. In 1363 the existing castle was integrated into the city fortifications

From 1486 to 1505, the Bishops of Augsburg expanded the Gothic castle into a High Castle, used as the summer residence of the church princes.