He was born in Fossombrone in 1682. From the age of 13 he attended the Collegio Clementino, of the Somaschi fathers, in Rome, driven by an intellectual curiosity that the province could not satisfy, but also by the ambition of his father, a nobleman from Forsemprone who was willing to invest heavily in his son’s ingenuity for which he hoped to at least obtain the purple. In 1701 at the end of his studies, as a test of his maturity Dominic held a public dispute on 300 philosophical theses in the courtyard of the College. The theses were published in the same year with a dedication to Pope Clement XI. From his first stay in Rome the young Passionei devoted himself to the search for ancient manuscripts and rare volumes that would help him in his studies so he was building up a valuable collection, which in 1705, already included 6,000 volumes.
It was not until 1721, after having already completed long diplomatic missions throughout Europe on behalf of the Pontiff, that Passionei decided to take his vows, later obtaining the nunciature in Switzerland and, in 1730 the same post in Vienna where he was official orator at the funeral of Prince Eugene of Savoy (1736).
Sojourns abroad were crucial to the young abbot’s education: in Paris he met the monks of St. Maurus-and Mabillon in particular-who, by verifying the authenticity of ancient manuscript documents, were attempting to implement the project of a scientific approach to history. In Holland, where an important book market was linked to a busy publishing business, he made contact with all the major booksellers, who would go on to receive from him and send him in turn book lists and valuable instructions for antiquarian research.
His constant travels around Europe gave him the opportunity to deepen his intellectual inclinations and to recognize himself in those circles that saw culture as a cosmopolitan phenomenon capable of bringing together, in a single republic of letters, men from various countries united by a common feeling of tolerance and cultural openness. These groups often identified themselves as anti-Jesuit groups in the name of open literary debate and the autonomy of scientific thought.
In 1738 he was appointed secretary of the Brevi, and in the same year he obtained the appointment as cardinal with the title of San Lorenzo in Lucina; he alternated his Roman sojourns with long periods of retreat in the sumptuous hermitage he had built for himself on the slopes of the Tusculum where he used to meet with the group of Roman Jansenists.
In 1741 he was appointed deputy librarian of the Vatican library under Cardinal Angelo Maria Querini, whom he succeeded in 1755. During this assignment he devoted himself to the recovery and restoration of many old books.
After his death in 1761 when his heirs decided to sell his library of about 50,000 printed works and 500 manuscripts, Pope Clement XIII did not want it to be taken out of Rome and convinced the General of the Augustinians Francisco Xaverio Vasquez to buy it and reunite it with the Angelica.