Girolamo Rusticucci was born in Cartoceto in 1537, the son of Ludovico Rusticucci, a famous jurisconsult, and his wife Diamante Leonardi. He was orphaned as a young man. As a young man, he studied literature and oratory.
Rusticucci traveled to Rome in 1557, entering the court of Cardinal Michele Ghislieri, who later became Pope Pius V, as the cardinal's personal secretary. When the cardinal became pope, he made Rusticucci a protonotary apostolic. Also, when Cardinal Michele Bonelli, Pope Pius V's cardinal-nephew, was absent, the pope put Rusticucci in charge of managing most of the church's affairs.
On June 16, 1570, he was elected Bishop of Senigallia. He was consecrated as a bishop in the Sistine Chapel by Cardinal Marcantonio Maffei. As a bishop, he promoted educating his clergy along the lines provided by the Council of Trent. He resigned the government of the diocese sometime before November 29, 1577.
Pope Pius V then named him papal legate to Spain and the Kingdom of France to promote war against the Ottoman Empire. He participated in the papal conclave of 1572 that elected Pope Gregory XIII, and in the papal conclave of 1585 that elected Pope Sixtus V. He served as Vicar General of Rome from 1588 to 1603. He was also a participant in the papal conclave of September 1590 that elected Pope Urban VII; the papal conclave of October–December 1590 that elected Pope Gregory XIV; and the papal conclave of 1591 that elected Pope Innocent IX. He became Camerlengo of the Sacred College of Cardinals on January 8, 1590 and was confirmed in the post on February 14, 1592. He participated in the papal conclave of 1592 that elected Pope Clement VIII. The new pope confirmed him as Camerlengo of the Sacred College of Cardinals, and he held that position from February 14, 1592 to 1593.
On August 18, 1597, he opted for the titular church of Santa Maria in Trastevere. Soon thereafter, he became the cardinal protopriest. He opted for the order of cardinal bishops on March 30, 1598, taking the Suburbicarian Diocese of Albano. He opted for the Suburbicarian Diocese of Sabina on February 21, 1600, and then for the Suburbicarian Diocese of Porto e Santa Rufina on February 19, 1603. He was the Vice-Dean of the College of Cardinals.
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