Costaguti was born in 1612 in Rome to the Costaguti;
Genoese nobility. He was the son of Prospero Costaguti (
Marquis of
Sipicciano and of Rocca Elvezia) and his first wife Paola Costa. The Costaguti family were responsible for commissioning the
Palazzo Costaguti in
rione Sant'Angelo, Rome which includes artistic works by
Domenichino,
Lanfranco and
Badalocchio. His younger half-brother,
Giambattista Costaguti, also later became a cardinal. At the age of 20 he went into the service of the Church and was appointed a
protonotary apostolic participantium and later was appointed as a regent of the
Apostolic Chancery under
Vice-Chancellor Cardinal
Francesco Barberini. Thereafter he was appointed
Commissary General of
Umbria and
Marca before obtaining a doctorate
utroque iure in 1638 which qualified him to be appointed
referendary of the
Tribunals of the Apostolic Signatura of Justice and of Grace. For a brief period in 1643 he was appointed Vice-
legate of
Ferrara. ==Cardinalate==