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Born in Florence, son of Piero Ridolfi and Contessina de' Medici (the daughter of Lorenzo de' Medici the Magnificent), he was therefore nephew of Pope Leo X, who granted him a quick ecclesiastical career. He was named governor of Spoleto in the period 1514-1516 and protonotary apostolic at the age of thirteen.
Pope Leo X created him cardinal deacon in the consistory of July 1, 1517 at the age of sixteen with the deaconry of SS. Vito e Modesto. Later his uncle appointed him administrator of the see of Orvieto on August 24, 1520 and he kept that post until September 3, 1529. Ridolfi participated in the conclaves of 1521–1522 and 1523.
Pope Clement VII elected him archbishop of Florence on January 11, 1524. He resigned from that position on October 11, 1532. He served also as administrator of Vicenza from March 14, 1524 until his death, administrator of Forli (April 16, 1526 - August 7, 1528). During the Sack of Rome (1527) he was taken hostage to Hugo of Moncada with other cardinals. Later he was named administrator of Viterbo (November 16, 1532 – June 6, 1533), administrator of the metropolitan see of Salerno (February 7, 1533- December 19, 1548) and administrator of Imola (August 4, 1533 - May 17, 1546). Pope Clement VII opted him for the deaconry of Santa Maria in Cosmedin on January 19, 1534. He participated in the Papal conclave, 1534.
Pope Paul III appointed him administrator of Viterbo again (August 8, 1538 - May 25, 1548) and opted him for the deaconry of Santa Maria in Via Lata on May 31, 1540] as he became cardinal protodeacon. He was a member of a special commission of eleven cardinals for reform of the Roman Curia. On January 8, 1543 he was named Archbishop of Florence for second time and resigned again on May 25, 1548.
After the death of Pope Paul III he was a papabile, entered the conclave of 1549 - 1550, but left because of illness. He died on January 31, 1550 of an apoplexy before the new Pope Julius III was elected on February 7, 1550. Cardinal Ridolfi was buried in the church of Sant'Agostino.
- Byatt, Lucinda M. C. (1988). "The concept of hospitality in a cardinal's household in Renaissance Rome". Renaissance Studies,. 2 (2): 312–320 – via JSTOR. (registration required (. )) [very tangential, of background value only]
- Byatt, Lucinda (1984). "Il Cardinale Niccolo Ridolfi ed il Palazzo di Bagnaia". Biblioteca e società. 4: 3–8. Retrieved 2016-05-09.
-  Una suprema magnificenza :Niccolo' Ridolfi a Florentine Cardinal in sixteenth-century Rome, by Lucinda Byatt, Ph.D. thesis, European University Institute, 1983.
- Ridolfi, Roberto (1929). "La biblioteca del cardinale Niccolo Ridolfi (1501-1550)". La Bibliofilia. 31: 173–193.