Maidalchini lahir pada 12 April 1631 di Viterbo, sebagai putra dari Andrea Maidalchini dan Pacifica Feliziani. Ayahnya merupakan saudara dari Olimpia Maidalchini; saudara ipar dari Paus Innosensius X.
- ^ The majority of the sources consulted (George L. Williams: Papal Genealogy: The Families And Descendants Of The Popes McFarland, 2004; Eleanor Herman: Mistress of the Vatican: The True Story of Olimpia Maidalchini: The Secret Female Pope, William Morrow, 2008 ISBN 0-06-124555-0; and Ludwig von Pastor: History of the Popes. Vol. 30. London: 1940, p. 39) agree that he was in his 17th year when promoted to the cardinalate. S. Miranda: Consistory of October 7, 1647, citing Berton's Dictionnaire des cardinaux, says that he was born on 21 April 1621. However, on the other place Miranda himself remarks that Berton's work should be used with the great caution because of the numerous errors it contains. Lorenzo Cardella, Memorie storiche de' cardinali della Santa Romana Chiesa, vol. VII, Rome 1793, p. 80 explains that some sources give 1621 as the year of his birth but this goes back to the typographical error in the renewed edition of Ciacconius' Vitae et res gestae Pontificum Romanorum et S.R.E. Cardinalium (1621 instead of 1631).
- ^ Contemporary, John Bargrave, states he was no older than 18 when he was made a cardinal, giving credence to the accuracy of a 1631 birth.
- ^ S. Miranda: Francesco Maidalchini