In the early Middle Ages, Mainz was a centre for the Christianisation of the German and Slavic peoples. The first Archbishop in Mainz, Boniface, was killed in 754 while trying to convert the Frisians to Christianity and is buried in Fulda. Other early archbishops of Mainz include Rabanus Maurus, the scholar and author, and Willigis (975–1011), who began construction on the current building of the Mainz Cathedral and founded the monastery of St. Stephan.
From the time of Willigis until the end of the Holy Roman Empire in 1806, the Archbishops of Mainz were archchancellors of the Empire and the most important of the seven Electors of the German emperor. Besides Rome, the diocese of Mainz today is the only diocese in the world with an episcopal see that is called a Holy See (sancta sedes). The Archbishops of Mainz traditionally were primas germaniae, the substitutes of the Pope north of the Alps.