Fabio Mignanelli was born in Siena ca. 1486, the son of Pietro Paolo Mignanelli and Onorata Saraceni.^[1] He attended the University of Siena, becoming a doctor of both laws.^[1] He then became a professor of law at the University of Siena.^[1]

In 1533, he moved to Rome, becoming a consistorial advocate.^[1] He was married to Antonina Capodiferro, the sister of Cardinal Girolamo Recanati Capodiferro and had a son.^[1] After Antonia died, Mignanelli entered the church.^[1] In 1537, he was sent as ambassador to the Republic of Venice to encourage Venice to break its alliance with the Ottoman Empire and to go to war against the Ottomans.^[1] In October 1537, Pope Paul III sent him as ambassador to Charles V, Holy Roman Emperor to present him with the brief postponing the Council of Trent for a second time.^[1] In spring 1538, he traveled to Nice with the pope to negotiate the end of the Italian War of 1536–1538; shortly thereafter, they traveled to Geneva to meet the emperor.^[1] On 3 September 1538 he was appointed nuncio to Ferdinand, King of the Romans, holding this position until 1539.^[1] In 1540, he became an auditor of the Roman Rota.^[1] He was also a protonotary apostolic and the pope's domestic prelate.^[1]

On 15 November 1540 he was elected Bishop of Lucera.^[1] He was appointed governor and vice-legate in Bologna on 14 April 1541.^[1] From 22 July 1543 to 2 August 1544 he was nuncio to the Republic of Venice.^[1] He was present at the opening of the Council of Trent on 13 December 1545.^[1] In 1546 he was vice-legate in Marche. On 20 September 1546 the pope sent him as nuncio to the emperor.^[1] In 1548, he was vice-legate in Ascoli Piceno.^[1] On 10 June 1551 he was appointed commissary of the Adriatic ports.^[1]

Pope Julius III made him a cardinal priest in the consistory of 20 November 1551.^[1] He received the red hat and the titular church of San Silvestro in Capite on 4 December 1551.^[1] Shortly thereafter, he became Prefect of the Apostolic Signatura.^[1] On 13 August 1552 the pope made him legate a latere to pacify Siena.^[1] On 17 May 1553 he resigned the government of his diocese, becoming administrator of Grosseto; he resigned in favor of his nephew, Giacomo Mignanelli on 2 October 1553.^[1] Pope Julius III then made him Prefect of the Papal States.^[1]

He was a participant in both the papal conclave of April 1555 that elected Pope Marcellus II and the papal conclave of May 1555 that elected Pope Paul IV.^[1]

He opted for the titular church of Santi Giovanni e Paolo on 12 June 1556.^[1]

He died in Rome on 10 August 1557.^[1] He was buried in Santa Maria della Pace.^[1]