A native of Montefano, a small village near Macerata and Loreto[1] he was the son of Ricardo Cervini who was the Apostolic Treasurer in Ancona.

[4] 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.

He continued to serve in that role throughout the remainder of Paul III's papacy, after which he was replaced to placate the Holy Roman Emperor Charles V (1519–56).

He was credited with defending not only orthodoxy and Church discipline, but also the universal claims of the Papacy in spiritual and temporal affairs, and with such vigor that the Emperor was affronted.

[10] The Apostolic Brief of his appointment, however, came from the new pope, Julius III, on 24 May 1550, and in it he was named not Vatican Librarian, but Bibliothecarius Sanctae Romanae Ecclesiae because he was the first cardinal to be placed in charge of the library.

[12] In the conclave of 1549–50 held to elect a successor to Paul III, fifty-one cardinals, including Marcello Cervini, participated at the opening on 3 December 1549.

The initial candidates included Cardinals Reginald Pole, Francesco Sfondrati, Rodolfo Pio da Carpi and Niccolò Ridolfi (who died on the night of 31 January).

Juan Álvarez de Toledo, Bishop of Burgos, another Imperial favorite, was proposed, and he too failed, because of strong opposition from the faction of Cardinal Alessandro Farnese, nephew of the late Pope Paul III and from the French.

[15] On 9 April 1555, on the evening of the fourth day of the papal conclave, Cervini was "adored" as pope, despite efforts by cardinals loyal to Emperor Charles V to block his election.

[16] Though Marcellus II desired to reform many of the inner workings of the Church, his feeble constitution succumbed to the fatigues of the conclave, the exhausting ceremonies connected with his accession, the anxieties arising from his high office, and overexertion in his performance of the pontifical functions of the Holy Week and Easter.

[18] When the Spanish ambassador asked for pardon for having killed a man, Marcellus replied that he did not want to start his reign with such auspices as absolution from homicide, and ordered the appropriate tribunals to observe the law.

He also gave audience to four cardinals, Farnese, D'Este, Louis de Guise and Ascanio Sforza, the leaders of the French faction in the recent conclave.

[5] Palestrina's Missa Papae Marcelli (dating from 1565 or before[19]), one of the glories of polyphonic sacred choral music, is traditionally believed to have been composed in his memory, ca.