During the post-Roman centuries of Germanic migration and settlement, the region afterwards included in the Diocese of Bamberg was inhabited for the most part by Slavs. The town, first mentioned in 902, grew up by the castle (Babenberch) which gave its name to the Babenberg family. On their extinction it passed to the Saxon house. The area was Christianized chiefly by the monks of the Benedictine Fulda Abbey, and the land was under the spiritual authority of the Diocese of Würzburg.
In 1007, Holy Roman Emperor Henry II made Bamberg a family inheritance, the seat of a separate diocese. The emperor's purpose in this was to make the Diocese of Würzburg less unwieldy in size and to give Christianity a firmer footing in the districts of Franconia, east of Bamberg. In 1008, after long negotiations with the Bishops of Würzburg and Eichstätt, who were to cede portions of their dioceses, the boundaries of the new diocese were defined, and Pope John XVIII granted the papal confirmation in the same year. Henry II ordered the building of a new cathedral, which was consecrated May 6, 1012. The church was enriched with gifts from the pope, and Henry II had it dedicated in honor of him. In 1017 Henry II also founded Michaelsberg Abbey on the Michaelsberg ("Mount St. Michael"), near Bamberg, a Benedictine abbey for the training of the clergy. The emperor and his wife Cunigunde gave large temporal possessions to the new diocese, and it received many privileges out of which grew the secular power of the bishop. Pope Benedict VIII during his visit to Bamberg (1020) placed the diocese in direct dependence on the Holy See. For a short time Bamberg was the centre of the Holy Roman Empire. Henry and Cunigunde were both buried in the cathedral.
From the middle of the 13th century onward the bishops were princes of the Empire and ruled Bamberg, overseeing the construction of monumental buildings. In 1248 and 1260 the see obtained large portions of the estates of the Counts of Meran, partly through purchase and partly through the appropriation of extinguished fiefs. The old Bishopric of Bamberg was composed of an unbroken territory extending from Schlüsselfeld in a northeasterly direction to the Franconian Forest, and possessed in addition estates in the Duchies of Carinthia and Salzburg, in the Nordgau (the present Upper Palatinate), in Thuringia, and on the Danube. By the changes resulting from the Reformation, the territory of this see was reduced nearly one half in extent.
The witch trials of the 17th century claimed about one thousend victims in Bamberg, reaching a climax between 1626 and 1631, under the rule of Prince-Bishop Johann Georg II Fuchs von Dornheim. The famous Drudenhaus (witch prison), built in 1627, is no longer standing today; however, detailed accounts of some cases, such as that of Johannes Junius, remain.
In 1647, the University of Bamberg was founded as Academia Bambergensis. Bambrzy (Ger. Posen Bambergers) – German Poles are descendants of settlers from the area near Bamberg, who settled in villages around Posen in the years 1719–1753. In 1759, the possessions and jurisdictions of the diocese situated in Austria were sold to that state. When the secularization of church lands took place (1802) the diocese covered and had a population of 207,000. Bamberg thus lost its independence in 1802, becoming part of Bavaria in 1803.
Bamberg was first connected to the German rail system in 1844, which has been an important part of its infrastructure ever since. After a communist uprising took control over Bavaria in the years following World War I, the state government fled to Bamberg and stayed there for almost two years before the Bavarian capital of Munich was retaken by Freikorps units (see Bavarian Soviet Republic). The first republican constitution of Bavaria was passed in Bamberg, becoming known as the Bamberger Verfassung (Bamberg Constitution).
In February 1926 Bamberg served as the venue for the famous Bamberg Conference, convened by Adolf Hitler in his attempt to foster unity and to stifle dissent within the young NSDAP. Bamberg was chosen for its location in Upper Franconia, reasonably close to the residences of the members of the dissident northern Nazi faction but still within Bavaria.
In 1973, the town celebrated the 1000th anniversary of its founding.