St. John's Church, then St. Salvator, consecrated in 911 by Archbishop Hatto I, served as the cathedral for the Bishop of Mainz until the appointment of Willigis as Archbishop of Mainz in 975.During Willigis' time, the city of Mainz flourished economically. Willigis became one of the most influential politicians of that time, he even was regent of the empire between 991 and 994. He ordered the construction of a new cathedral in the pre-Romanesque Ottonian architecture. This new and impressive building was part of Willigis' vision of Mainz as the "second Rome".
This new cathedral was to take over the functions of two churches: the old cathedral and St. Alban's, which was the largest church in the area, belonging to a Benedictine abbey and serving as the burial ground for the bishops and other nobles, including Fastrada, a spouse of Charlemagne. Most of the synods and other important meetings were held at St. Alban's Abbey.
The new cathedral consisted of a double chancel with two transepts. The main hall was built in the typical triple-nave "cross" pattern. As was usual at that time no vault was included because of structural difficulties relating to the size of the building. Six towers rose from the church. A cloister was enclosed in the structure and a small freestanding church, St. Mary's Church, connected by a colonnade. This small church developed later into the collegiate church of St. Maria ad Grada.