Doria studied philosophy and theology in Spain being promoted to the Cardinalate at the instance of King Felipe II of Spain. Once king Philip II died, he was created cardinal deacon, aged 31, in the consistory of June 9, 1604. Doria was granted permission to receive the sacred orders outside the Ember days, December 9, 1604. He participated in both the March–April 1605 papal conclave, which elected Pope Leo XI, and in the May 1605 papal conclave, which elected Pope Paul V.