He made his way up through the clerical ranks as Referendary of the Tribunals of the Apostolic Signature of Justice and of Grace (1580), and later went to serve the grand-duke of Tuscany, the former Cardinal Ferdinando de' Medici.
[3] Academics such as Posner, Frommel, and Hibbard have drawn upon extant documents (principally the correspondence of Dirk van Ameyden) that suggest the strong likelihood that he was homosexual[4] and this may have influenced his tastes in the art he commissioned (including those by Caravaggio), as well as damaging prospects of assuming the papacy.
But Graham-Dixon argues that such accusations seem deliberately to have been cast by the pro-Spanish Ameyden against the pro-French Del Monte in order to discredit him, and bear little real scrutiny.
[7] Del Monte was a perceptive supporter of the arts and sciences – he was the first recorded owner of the Portland Vase, and his Palazzo Madama household was one of the most important intellectual salons in Rome.
At his death his art collection contained more than six hundred paintings, and his support of the young Caravaggio has given provenance to several of that artist's early works.