[4] Marco became Maestro di casa of his cousin the cardinal, and in 1449 became his procurator for the Premonstratensian abbey of Sainst Severo e Martirio near Orvieto.

After a detailed report, featuring the financial state of the Knights, was presented, Paul II issued a bull "Quamvis ex Commisso" on 14 February 1466, embodying an analysis of the problems and the outlines of a reform of the Order.

[13] Barbo was appointed a cardinal by Pope Paul II on 18 September 1467, and assigned the titular church of San Marco on 2 October 1467.

[18] On 22 December 1471 Barbo was made legate to Germany, Hungary and Poland by Pope Sixtus IV, to summon the Emperor Frederick III and the Christian people in his domains to combat the Ottoman Turks.

Possessed of several abbacies in commendam, including the monastery of Santa Croce di Sassovivo from 14 July 1476,[21] he was elected Camerlengo of the Sacred College of Cardinals for the year 1478.

[25] During the consistory, Barbo was one of those considered papabile, and he received ten votes on the first scrutiny; he had an enemy in Cardinal Rodrigo Borgia, however, who was opposed to his candidacy.

Barbo was an erudite patron of the humanists so distrusted by Paul II, but as Commissary-Administrator[27] of the Sapienza, he was constrained to withhold the salary of Pomponio Leto, who had fled to Venice.