The original walls of the Merovingian basilica are still preserved up to a height of 16 meters. This has been confirmed by recent excavations and building research. Since then, the investigations in Mainz have attracted a great deal of public interest.

The old Mainz Cathedral was built in the 7th century, partly on the walls of a previous building from the 4th/5th century. The cathedral was designed as a three-nave basilica with a double choir and a west transept. With a 13-meter-wide central nave, it was a monumental building for its time.

With the construction of the new cathedral (Willigisdom) in 1036, it was converted into a collegiate church. The west transept was demolished and the side aisles were extended to visually lengthen the short nave. The west choir was built in the 14th century. At around the same time, a rood screen was built, the base and foundations of which were uncovered during recent excavations. After Baroque reconstruction (demolition of the rood screen and raising of the floor level), the church was initially used as a straw store and military depot during the French period. It was then transferred to the Protestant community (1828), which converted the building into a Protestant worship space with an organ, central pulpit, and galleries in the nave. In 1906, the interior was renovated again in Darmstadt Art Nouveau style by architect Friedrich Pützer. After an air raid in 1942, the church burned down. Following extensive renovation by architect Karl Gruber, St. Johannis was re-consecrated in 1956 in a simple, historicist style. Comprehensive research into the building's history has been ongoing since 2013.

Due to the excavation work, the church is currently closed. Visits are only possible by prior arrangement. Telephone: +49 6131 9205886.