He served as an official in the Vicariate of Rome from 1929 to 1939, when he was made a referendary of the Apostolic Signatura on 23 April.

[1][2] On 14 December 1958, Ferretto was appointed Titular Archbishop of Serdica by Pope John XXIII.

He received his episcopal consecration in St. Peter's Basilica on the following 27 December from Pope John, with Bishops Girolamo Bortignon, OFM Cap and Gioacchino Muccin serving as co-consecrators.

He was created cardinal priest of Santa Croce in Gerusalemme by Pope John in the consistory of 16 January 1961.

While the older method had ensured that only senior clerics of advanced age held the title of cardinal bishop, Pope John's first appointment under the new rule was Ferretto, who had just turned 62.