Born in Carpineto Romano, near Rome, Giuseppe was one of the seven children of Count Dominico Ludovico Pecci (2 June 1767 – 8 March 1833), Patrician of Siena, Colonel of the French Army under Napoleon, and his wife Anna Francesca Prosperi-Buzzi (1773 – 9 August 1824).

In 1828, the question of occupational choice arose for the two brothers; Giuseppe Pecci professed the Jesuit order, while Vincenzo decided in favour of a diocesan priest.

Good Thomist theology was hard to come by at that time, with the result that young scholars from other countries were sent to Rome to learn from Pecci and Tommaso Maria Zigliara.

After the Papal speech, each of the new cardinals, Pecci, John Henry Newman, Joseph Hergenröther and Tommaso Maria Zigliara, received the red hat, all of whom being well-known Church scholars.

)[15] Pope Leo XIII appointed thirty members, ten each from Rome, from Italy, and from the rest of the world, and provided generous financial support to attract scholars from everywhere.

[14] To balance his Thomist Jesuit appointments, Leo entrusted the overall responsibility of the works of St Thomas Aquinas to the Dominican Order, of which the saint had been a member.

Pope Leo XIII considered the mostly locked-up and neglected Vatican Library "an infinite treasure for the Church and a monument to its role in culture and science".

[16] He greatly increased staff and organization and appointed Jesuit father Franz Ehrle and Giuseppe Pecci to head the new undertaking as prefect and librarian, respectively.