[4] His dissertation was titled "Die kirchlichen Reunionsbestrebungen während der Regierung Karls V" (The Church's Attempts at Reunion During the Reign of Charles V).

Pastor edited his mentor Janssen's eight-volume Geschichte des deutschen Volkes (History of the German People) and published it from 1893 to 1926.

Pastor consulted archives throughout Catholic Europe and, during his first trip to Italy in 1881, his seriousness ensured the patronage of Pope Leo XIII, who opened to him the contents of the Vatican Library, which had previously been held unavailable to scholars.

[3] He was granted privileged access to the Secret Vatican Archives, and his history, largely based on hitherto unavailable original documents, superseded all previous histories of the popes in the period he covered, which runs from the Avignon Papacy of 1305 to Napoleon's entrance in Rome, 1799.

The result of his research was his Geschichte der Päpste seit dem Ausgang des Mittelalters in sixteen volumes.

The opus magnum was subsequently translated into English and published as History of the Popes From the Close of the Middle Ages.