On account of the services of the bishops to the Merovingian kings, the city was to have been the metropolitan see of Saint Boniface, but Mainz was chosen, for unknown reasons, and Cologne did not become an archbishopric until the time of Charlemagne.

The city suffered heavily from Viking invasions, especially in the autumn of 881, but recovered quickly from these calamities, especially during the reign of the Ottonian emperors.

[2] From the mid-13th century, the Electorate of Cologne—not to be confused with the larger Archdiocese of Cologne—was one of the major ecclesiastical principalities of the Holy Roman Empire.

In 1821, the archdiocese regained Cologne and the right bank of the Rhine (though with a new circumscription reflecting the Prussian subdivisions) and, in 1824, an archbishop was established there again.

Cologne, the largest (in terms of inhabitants non-Catholics included) and reportedly richest diocese in Europe, announced in October 2013 that "in connection with the current discussion about Church finances" that its archbishop had reserves amounting to 166.2 million Euro in 2012.