St. Johannis Church in Mainz, traditionally revered as the city's "Old Cathedral," stands as a foundational monument in Germany's religious history. Recognized as the oldest church in Mainz, it represents one of the earliest episcopal churches established north of the Alps. Its historical foundations reach back exceptionally far, with evidence suggesting a Christian church was established here as early as the 5th or 6th century, constructed upon the site of late antique structures. This remarkable continuity makes it the only surviving church building from the Merovingian period in the country. Its origins are traced to the 5th or 6th century, establishing it as a critical site from the dawn of Christianity in the region.

The church’s primary construction phase began in the 7th century, establishing it as the first episcopal seat of the Bishop of Mainz. Throughout the early Middle Ages, it developed from a Merovingian foundation into a representative Ottonian basilica in the 10th century, functioning as a major spiritual and political center. Notably, figures such as Boniface and Willigis are documented to have worked within its walls. The site’s significance is further highlighted by its role in two royal coronations during the 11th century.

After the new Mainz Cathedral was completed in 1036, St. Johannis transitioned to a collegiate church. Over the subsequent centuries, its architecture was enriched by a Gothic choir added in the 14th century. Significant architectural changes were introduced over time, including a double chancel design in the Carolingian era. Subsequent significant modifications included a late Baroque renovation in the 18th century. Its later history reflects the broader political shifts in Europe. Following the Napoleonic era, the church was reassigned to the Protestant community in 1828.

Following this transition, the church was severely damaged during World War II, leading to a major restoration in the 1950s that secured its structure for future generations.

The architectural character of St. Johannis is predominantly Carolingian, though subsequent additions introduced a variety of architectural influences.

The building itself is a basilica featuring a wide central nave that rises above its narrower side aisles. Renewed archaeological interest since 2013 has uncovered profound historical layers and artifacts, confirming its role as the cathedral of the Mainz archbishopric before the current cathedral was built.

Recent archaeological work has been particularly illuminating. One of the most significant discoveries came in 2019 with the unsealing of a sarcophagus believed to contain the remains of Archbishop Erkanbald, who was buried there in 1021. Archaeologists have identified a total of seven distinct floor levels. The current ground level stands approximately 2.6 meters higher than it was during the time of Archbishop Hatto I, who oversaw a major renovation.

The excavations have revealed not only a floor from the 9th century but also evidence of a significant structure dating back even further to the early Carolingian period, possibly the 7th or 8th century. The most striking find was the uncovering of a sarcophagus believed to contain the remains of Archbishop Erkanbald. The investigations also uncovered wall remnants suggesting an earlier building, which could have been an early Christian community building or even a Roman-era structure beneath the church.