[1] He was made a cardinal in 1593 by his uncle, Pope Clement VIII, with the title Cardinal-deacon of San Nicola in Carcere.

In 1600, he traveled to Lyon as papal legate to bless the marriage of Henry IV of France and Marie de' Medici.

[3][4] He became archbishop of Ravenna in 1604 and Cardinal-Priest of San Pancrazio, but opted the following year for Santi Giovanni e Paolo al Celio.

In 1598, Pope Clement gave Pietro the old Ruffini villa at Frascati as a reward for successful negotiations he had undertaken with the French.

The upper floors of the Palazzo Aldobrandini held a rich collection of works of art left to the cardinal by the Duchess of Urbino Lucrezia d'Este, in 1598.