Cesare Baronio was born at Sora in Italy in 1538 as the single child of Camillo Baronio and Porzia Febonia.
He was educated at Veroli and Naples where he commenced his law studies in October 1556. At Rome he obtained his doctorate in canon law and civil law. After this he became a member of the Congregation of the Oratory in 1557 under Philip Neri - future saint - and was ordained to the subdiaconate on 21 December 1560 and later to the diaconate on 20 May 1561. He was then ordained to the priesthood in 1564. He succeeded Neri as superior in 1593.
Pope Clement VIII - whose confessor he was from 1594 - elevated him into the cardinalate on 5 June 1596 and also appointed him as the Librarian of the Vatican. Baronio was given the red hat on 8 June and received status as Cardinal-Priest of Santi Nereo e Achilleo on 21 June.
Baronius restored his titular church of Church of Sts Nereus and Achilleus and a procession in 1597 celebrated a transfer to it of relics. He also had work done on the Church of San Gregorio Magno al Celio.
At subsequent conclaves Baronio was twice considered to be papabile - the conclaves saw the elections of Pope Leo XI and Pope Paul V. On each occasion he was opposed by Spain on account of his work "On the Monarchy of Sicily", in which he supported the papal claims against those of the Spanish government.