 Simeoni was published as Cardinal Priest of "San Pietro in Vincoli" in the consistory of September 17, 1875.Between December 18, 1876 and the death of Pope Pius IX on February 7, 1878, he also served as Vatican Secretary of State, Prefect of the Sacred Congregation of Public Ecclesiastical Affairs, Prefect of the Apostolic Palace, and Administrator of the Patrimony of the Holy See; Pius IX had designated him as executor of his will as well.The Cardinal then participated in the conclave of 1878, which resulted in the election of Pope Leo XIII, who confirmed him as Prefect of the Apostolic Palace and Administrator of the Patrimony of the Holy See.