Ginetti was born in Velletri, the son of a labourer. He was sent to Rome at a very young age to be educated and tried to make himself known to a number of cardinals. Eventually he was appointed chamberlain to Pope Paul V. Upon that Pope's death, Maffeo Barberini was elected as Pope Urban VIII and Ginetti became acquainted with Cardinal Francesco Barberini, the new Pope's nephew. Barberini urged his uncle to promote Ginetti and he was made a Referendary of the Tribunals of the Apostolic Signatura. Thereafter he remained a loyal supporter of the Barberini.
On 19 January 1626 he was elevated to cardinal by Pope Urban VIII and installed as Cardinal-Priest at Santa Francesca Romana in 1627. In 1629 he was made Cardinal Vicar (a title he held until his death) and was made Cardinal-Priest at Sant'Angelo in Pescheria in 1634.
In 1635 Ginetti was appointed Papal Legate to Austria in an attempt, by the Barberini, to put him forward as an elder statesman. But he did not conduct himself particularly well and the effort was seen as counter-productive. Ginetti returned to Rome where in 1639, he and Cardinal Bernardino Spada gave assistance and protection to a visiting Carlo Cesare Malvasia.
In 1644 he was appointed Cardinal-Priest at Sant'Eustachio and participated in the papal conclave of that year which elected Pope Innocent X. Contemporary, John Bargrave, suggested Ginetti considered himself papable; suitable for election to the papacy. But the Barberini would not entertain it and any efforts on Ginetti's part to suggest as much came to naught.
- Pope Alexander the Seventh and the College of Cardinals by John Bargrave, edited by James Craigie Robertson (reprint; 2009)
- Malvasia's Life of the Caracci by Carlo Cesare Malvasia (autobiography), edited by Anne Summerscale (Penn State Press, 2000)
- Catholic Hierarchy: Marzio Ginetti
- S. Miranda: Marzio Ginetti