He succeeded Pope Julius III. Before his accession as pope he had been Cardinal-Priest of "Santa Croce in Gerusalemme". He is the most recent pope to choose to retain his birth name as his regnal name upon his accession, as well as only the second and most recent pope to date to take the name "Marcellus" upon being elected. After his death, it would be 423 years before another pope would choose a name with an ordinal number less than IV (John Paul I). Cervini was the maternal uncle of Robert Bellarmine. Cervini's father and Pope Clement VII were personal friends. Cervini served in the household of Cardinal Alessandro Farnese. When Farnese became Pope, Cervini served as his secretary and was employed on some diplomatic missions. On 10 April, 1555, he was elected to succeed Pope Julius III. He died of a stroke twenty-two days later. A native of Montefano, a small village near Macerata and Loreto he was the son of Ricardo Cervini who was the Apostolic Treasurer in Ancona. The family originated in Tuscany, in the town of Montepulciano, which had once been subject to Siena, but later was under the control of Florence. Marcello had two half-brothers, Alexander and Romulus. One of his sisters, Cinzia Cervini, married Vincenzo Bellarmino, and was the mother of Roberto Bellarmine. Marcello was educated locally, and at Siena and Florence, where he became proficient in writing Latin, Greek, and Italian. He also received instruction in jurisprudence, philosophy, and mathematics. His father had an interest in astrology and upon discovering that his son's horoscope presaged high ecclesiastical honours, Riccardo set the young Cervini on a path to the priesthood. After his period of study at Siena, Cervini traveled to Rome in the company of the Delegation sent by Florence to congratulate the new Pope on his election. His father and Pope Clement VII were personal friends, and Marcello was made "Scrittore Apostolico". He was set to work on astronomical and calendar studies, a project which was intended to bring the year back into synchronization with the seasons. In 1527, he fled home after the Sack of Rome, but eventually returned and was taken into the household of Cardinal Alessandro Farnese "senior".