Somewhat hidden between Münsterstraße and Walpodenstraße stands the Protestant Altmünsterkirche. The modern and simple building with its two white towers gives little indication of its long and eventful history. According to legend, the monastery known as "Altmünster" since the early Middle Ages was founded around 700 by "Bilehilt" (Saint Bilhildis), the niece of the then Bishop Rigibert, as a small convent for women.

It initially followed Benedictine rules, but was assigned to the Cistercian Order in the course of the church reform of the 13th century. The monastery had to leave its original location between Bingerstraße and Bahnhofstraße in the 17th century and was rebuilt 100 meters further south in the early Baroque style. Altmünster was originally richly furnished. The most important relic was the Holy Shroud of Christ, which was displayed on Easter Monday and is now kept in the cathedral.

In 1802, the Altmünster Church was handed over by the French prefect of the newly founded United Protestant Community in Mainz. The Jakobsberg Benedictines, who had been quartered in Altmünster in the meantime, had to vacate the church and monastery. For the first time since the Thirty Years' War, a Protestant service was celebrated again in Mainz in Altmünster. Only a few years later, the Protestants were forced to move to the small Welschnonnenkirche church on Napoleon's orders. The Altmünster monastery, which was under French military administration, first became a maternity hospital and then, from 1808, a military hospital. Altmünster continued to be used for military purposes in the following years: from 1895 as a Protestant garrison chapel and finally as a French Catholic garrison church.

In 1929, the Protestant community of Mainz bought back the church from the Reich Treasury for 88,000 Reichsmarks. The Altmünsterkirche was designated the third Protestant city church after St. Johannis Church and Christuskirche.

During the Nazi era, the church and the neighboring Wartburgheim were a center of the "German Christian Movement" and became a garrison church again in 1936. After the destruction of Altmünster in World War II, community life came to a complete standstill.

It was not until the end of the 1950s that the church was redesigned and rebuilt in the style of the time by building officer Otto Vogel. The decorative elements on the east facade were removed, the late Renaissance portal was demolished, the towers were redesigned, and the windows were reduced in size. The main entrance was moved to the west, facing Walpodenstraße.

The church was reconsecrated in 1960 and has since served as a place of worship for the Altmünster parish. Thanks to close cooperation with the music academy, there are two organs in the Altmünsterkirche: students practice here on weekdays, and the organs are played during church services on weekends and holidays.