"This cathedral, towering in all its might and grandeur above the Rhine plain, would have remained in my memory forever, even if I had never seen it again," wrote the authoress Anna Seghers. This massive cathedral has determined the course of Mainz's history and still gives Mainz its character to this day, a thousand years after its construction. Mainz, at the crossroads of the Old World's nations, became the ecclesiastical center north of the Alps as of 746/47, through the work of St. Boniface. It acquired the title of "Holy See" during the rule of Willigis (975-1011).

This archbishop, at the same time archchancellor of the Holy Roman Empire, began construction of the cathedral in 975, modelling it on old St. Peter's in Rome. Seven coronations took place in Mainz cathedral over the centuries. The new cathedral did not, however, survive the day of its consecration in August 1009, when a fire destroyed the building and the cathedral could only be used again in 1036. Willigis was therefore buried in St. Stephen's.

The bronze door panels of the market portal, the oldest surviving items of furnishing of the Romanesque pier basilica, date from his reign in the 10th century. The inscription on this ceremonial door refers to its constructor and the artist. The cathedral has three naves, two chancels, and numerous chapels. The west chancel with the main altar is dedicated to St. Martin, and the east chancel to St. Stephen.

Mainz Cathedral is still almost completely enclosed by surrounding buildings. The red tinted sandstone "mountain" of a cathedral, which grew over the centuries, contrasts with the light colored stone of the Romanesque St. Gotthard's Chapel, which Archbishop Adalbert erected before 1137 as the archbishops' private chapel. A crucifix from the period of the Hohenstaufen Emperors is preserved in this chapel. In 1778/79 Ignaz Michael Neumann, the son of the famous baroque architect Balthasar Neumann, built the cathedral houses on the Leichhof, the former burial grounds. As there was a great fear of fire, because the cathedral caught fire seven times over the centuries, these buildings were constructed with fire proof stone roofs. After being hit by lightning, the west tower was given a new spire with references to the Gothic belfry in its forms in 1767. This was, however, built in stone, and not as previously in timber.

The east chancel, with walls measuring over two meters in thickness, is the oldest part of the cathedral. Its crossing tower was partly destroyed in the 1793 bombardment of Mainz. It was rebuilt at the beginning of the 19th century. The city architect Georg Moller designed its round iron dome, which was later removed to be replaced by a more historic pointed roof.

The beautiful adorned capitals on the portals facing Liebfrauenplatz (around 1100) were sculpted by stone masons from Lombardy. In the 19th century, an 11th-century crypt was found under the east chancel.

The late Romanesque west chancel was constructed between 1200 and 1239. In the 15th century, a Gothic belfry was added to the crossing tower. The sale of the carved rococo choir stalls of 1767 was prevented by Bishop Joseph Ludwig Colmar, who was appointed by Napoleon. It was also Colmar who convinced the French Emperor not to demolish the cathedral after the secularisation of 1803. Since 1928, bishops have been buried in the new crypt under the west chancel. Of the 84 bishops and archbishops who have held office since Boniface, 45 are buried in the cathedral. A large number of tombstones and memorials from the 11th to the 20th centuries adorn the walls and pillars of the church and cloister.

The often idealised portrayals of the ecclesiastical dignitaries reflect the history of the diocese of Mainz. Their completeness and their good state of preservation make this portrait gallery one of the most important sights in the cathedral. Old wall paintings and stained glass windows have not survived. The wall paintings in the middle nave to designs by the painter Philipp Veit, who was regarded as a Nazarene, date back to the 19th century. The Lady altar in the Ketteler Chapel, with it's "beautiful Lady of Mainz" is a center of attraction. The late Gothic wooden sculpture (around 1510) is in the series of the wooden carvings by Hans Backoffen, from whose workshop three of the tombs in the cathedral came.

The 15th century cloister has two stories. In the past, the canons of the cathedral lived in the collegiate buildings. Today the Episcopal and Diocesan Museum is housed there, in which ecclesiastical treasures from many eras are preserved and displayed.