The Elector of Mainz was one of the seven Prince-electors of the Holy Roman Empire. As both the Archbishop of Mainz and the ruling prince of the territory, the Elector of Mainz held a powerful position during the Middle Ages. The Archbishop-Elector was president of the electoral college, arch-chancellor of the empire and primate of Germany until the dissolution of the empire in 1806.
The origin of the title dates back to 747, when the city of Mainz was made the seat of an archbishop, and a succession of able and ambitious prelates made the district under their rule a strong and vigorous state. Among these men were important figures in the history of Germany such as Hatto I, Siegfried III, Peter of Aspelt, and Albert of Mainz. There were several violent contests between rivals for the archbishopric, and their power struggles occasionally moved the citizens of Mainz to revolt. The lands of the elector lay around the city of Mainz on both banks of the Rhine; their area reached 3200 sq. miles by the end of the Empire. The last elector was Karl Theodor von Dalberg, who lost his temporal power when the archbishopric was secularized in 1803.
The Archbishop of Mainz was an influential ecclesiastic and secular prince in the Holy Roman Empire between 780-782 and 1802. In Church hierarchy, the Archbishop of Mainz was the primas Germaniae, the substitute for the Pope north of the Alps. Aside from Rome, the See of Mainz is the only other see referred to as a "Holy See", although this usage became rather less common.
This archbishopric was a substantial ecclesiastical principality of the Holy Roman Empire. The ecclesiastical principality included lands near Mainz on both the left and right banks of the Rhine, as well as territory along the Main above Frankfurt (including the district of Aschaffenburg), the Eichsfeld region in Lower Saxony and Thuringia, and the territory around Erfurt in Thuringia. The archbishop was also, traditionally, one of the Imperial Prince-Electors, the Arch-chancellor of Germany, and presiding officer of the electoral college technically from 1251 and permanently from 1263 until 1803.
The see was established in ancient Roman times, in the city of Mainz, which had been a Roman provincial capital called Moguntiacum, but the office really came to prominence upon its elevation to an archdiocese in 780/82. The first bishops before the 4th century have legendary names, beginning with Crescens. The first verifiable Bishop of Mainz was Martinus in 343. The ecclesiastical and secular importance of Mainz dates from the accession of St. Boniface to the see in 747. Boniface was previously an archbishop, but the honor did not immediately devolve upon the see itself until his successor Lullus.
In 1802, Mainz lost its archiepiscopal character. In the secularizations that accompanied the Reichsdeputationshauptschluss ("German mediatization") of 1803, the seat of the elector, Karl Theodor von Dalberg, was moved to Regensburg, and the electorate lost its left bank territories to France, its right bank areas along the Main below Frankfurt to Hesse-Darmstadt and the Nassau princes, and Eichsfeld and Erfurt to Prussia. Dalberg retained the Aschaffenburg area however, and when the Holy Roman Empire finally came to an end in 1806, this became the core of Dalberg's new Grand Duchy of Frankfurt. Dalberg resigned in 1813 and in 1815 the Congress of Vienna divided his territories between the King of Bavaria, the Elector of Hesse, the Grand Duke of Hesse-Darmstadt and the Free City of Frankfurt.
The modern Diocese of Mainz was founded in 1802, within the territory of France and in 1814 its jurisdiction was extended over the territory of Hesse-Darmstadt. Since then it has had two cardinals and via various concordats was allowed to retain the mediæval tradition of the cathedral chapter electing a successor to the bishop.