The Old Cathedral of St. John is considered the oldest church in Mainz and is among the earliest bishop's churches north of the Alps. It was the predecessor of the present Mainz Cathedral and developed over centuries into a significant spiritual and political center of the early Middle Ages.
A first Christian church facility was established at this site in the 5th or 6th century through the reconstruction of late antique buildings. Over the centuries, the structure was repeatedly expanded and reshaped—from an early medieval bishop's church with a baptismal house to the representative Ottonian basilica in the 10th century. Significant church figures such as Boniface and Willigis worked here, and two royal coronations took place in the 11th century.
After the construction of the new cathedral in 1036, St. John became a collegiate church. In the following period, the church underwent further renovations, such as the construction of a Gothic choir in the 14th century or in the 18th century in the style of the late Baroque. During the Napoleonic occupation, it served temporarily as a warehouse before being transferred to the ownership of the Protestant community in 1828. Since then, it has shaped Protestant life in Mainz.
The church was severely damaged in World War II but was restored in the 1950s. Archaeological investigations, which have been ongoing since 2013, have uncovered numerous historical layers and finds—including the grave of Archbishop Erkanbald from 1021 in 2019.
Today, the Old Cathedral is not only a significant architectural monument with visible traces from late antiquity to modern times but also a place of spiritual encounters, archaeological discoveries, and cultural events in the heart of Mainz.
