St. John's is the oldest intact church in Mainz and also the only preserved cathedral building from the late Carolingian and early Ottonian times (7th century) in Germany. Recent excavations revealed that, up to a height of about 16 metres, the current walls are those of a Carolingian basilica. Since this discovery, there is a great public interest in this building and its history.
In the 7th century, the first cathedral of Mainz was built. It was partly erected on foundations and walls of a previous church dating from the 4th and 5th centuries. The original cathedral was built as a basilica with wide central nave higher than the narrow aisles. It also featured an east-west transept. With its 13 metre wide nave, it was a truly monumental building for the time.
With the construction of the new cathedral at the time of Archbishop Willigis in 1036, St. John's became a collegiate church. The transept was demolished and the aisles were extended to make the relatively short nave appear longer. The west chapel dates from the 14th century. It is likely that it came with a chancel screen built during the same period whose pedestal was only recently excavated. Following a refurbishment of the church in the Baroque period (during which the screen was removed and the floor was elevated), the church was used for a while as a straw shed and army warehouse by the French occupying forces. In 1828, the church was handed over to the Protestant parish of Mainz and refurbished as a place of worship complete with organ, centre pulpit and galleries along the nave. In 1906, the interior was once more refurbished, this time by Friedrich Pützer in the Art Déco style. An air raid in 1942 set the church ablaze. After extensive renovation and refurbishment in an unobtrusive, historicising style by Karl Gruber, St. John's was reconsecrated in 1956. Since 2013, it has been the object of extensive building history research.