Once king Philip II died, he was created cardinal deacon, aged 31, in the consistory of 9 June 1604.

Doria was granted permission to receive the sacred orders outside the Ember days on 9 December 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. Doria received the red hat, the deaconry of S. Adriano, on 5 December 1605 and the position of Abbot commendatario of San Fruttuoso in Camogli.

[clarification needed] Doria participated in the 1621 papal conclave, which elected Pope Gregory XV.

On 2 October 1623 Doria opted for the order of cardinal priests and the title of San Pietro in Montorio, He celebrated the discovery of the relics of Saint Rosalia, and promoted her cult as patroness and protector of Palermo.