Pre-Roman

The Greek geographer Strabo mentions in 50 BC a town of the Celtic Estiones named Kambodunon. This is considered the oldest written reference of any German city. So far no archaeological evidence could be found that this Celtic settlement really existed.

Roman era

In 15 BC Roman troops led by Nero Claudius Drusus and his brother Tiberius conquered and destroyed an existing Celtic settlement. Later the settlement was named Cambodunum. In the following years the city was rebuilt on a classical Roman city plan with baths, forum and temples. Initially in wood, the city was later rebuilt in stone after a devastating fire that destroyed almost the entire city in the year 69 AD. The city possibly served as provincial capital of Raetia during the first century before Augsburg took over this role. Extensive archaeological excavations at the end of the 19th century and again during the 1950s at what were then the outskirts of Kempten unearthed the extensive structural foundations.

The city was again destroyed in 233 AD by the Alemanni, a Suebic tribe. The original site of Cambodunum was then abandoned and the settlement moved to a strategically safer location on the Burghalde hill overlooking the river Iller.

In the middle of the 5th century the last Roman troops had left the area and the city was entirely taken over by the Alemanni.

Middle Ages

After the Romans abandoned the settlement, it was moved from the hill down to the plains located next to the river Iller. In written sources, the town appears as Cambidano. Being still predominantly Alemannic, the town once more was destroyed by the Franks in 683 as a consequence of the city's support of an uprising against the Frankish kingdom.

Around 700 a monastery — Kempten Abbey — was built, the first in the Allgäu region, founded by two Benedictine monks from the Abbey of Saint Gall, Magnus von Füssen and Theodor.^[4] This new monastery's first abbot was one Audogar. Through the financial and lobbyist support of Charlemagne’s wife Hildegard, an Allemannic princess, the monastery came to be one of the most privileged of the Frankish Empire.

After the abbey had several times been ravaged by the Magyars, the Prince-Bishop of Augsburg, Ulrich of Augsburg, who was also Abbot of Kempten, began the rebuilding of both the monastery and the city in 941.

In 1213, the Holy Roman Emperor Frederick II declared the abbots members of the Reichstag and granted the Abbot of Kempten the right to bear the title of Duke. However in 1289, King Rudolf I of Germany also granted special privileges to the urban settlement in the river valley, making it the Free Imperial City of Kempten. In 1525 the last property rights held by the abbots in the Imperial City were sold in the so-called "Great Purchase", marking the start of the co-existence of two independent cities next to each other, each bearing the same name. More conflict arose in 1527 after the Imperial City converted to Protestantism in direct opposition to the Catholic monastery (and Free City).

Renaissance and Baroque to Modern Age

During the turmoil of the Thirty Years' War (1632–33), both cities were destroyed by the imperial forces and the Swedish troops respectively.

In 1652^[5] Roman Giel of Gielsberg, the Abbot of Kempten, commissioned the architects Michael Beer and Johann Serro from Graubünden to build St. Lorenz Basilica as a new church to serve the parish and monastery, including a representative residence for the Duke-Abbots. This is acknowledged as the first large church built in Germany after the end of the Thirty Years' War.

During the Napoleonic Wars the Dukedom-Abbey and Imperial City came under Bavarian rule (1802–03). Finally, in 1819, the two rival cities were united into a single communal entity.

The city was the location of two subcamps of the Dachau concentration camp during World War II, each with about 700 inmates.^[6][7]

Claude (Claudius) Honoré Desiré Dornier born in Kempten im Allgäu (May 14, 1884 - December 5, 1969) was a German aircraft engineer and founder of Dornier GmbH. His legacy remains in the few aircraft named after him, including the Dornier Do 18 and the 12-engine Dornier Do X flying boat, for decades the world's largest and most powerful airplane.