Mattei was born 8 February 1547, the son of Alessandro Mattei and Emilia Mazzatosta. He was the younger brother of
Ciriaco Mattei and the older brother of
Asdrubale Mattei,
Marquis di Giove. He was uncle to
Girolamo Mattei, Duca di Giove and
Luigi Mattei, commander of forces loyal to the Pope. Mattei was raised to cardinal deacon in 1586 with the
diaconias of
Sant'Adriano al Foro and
Sant'Agata dei Goti (1587) then
Santa Maria in Cosmedin (1587–89), then the titular church of
San Pancrazio fuori le mura from 1592 to 1603. He participated in the
conclaves of 10–30 January 1592 (electing
Pope Clement VIII) and also the elections of
Pope Innocent IX,
Pope Gregory XIV and
Pope Urban VII. From 1591 to 1603 he was prefect of the
Congregation of the Council. ==See also==