His ecclesistic career started with the appointment as abbot, in commendam, of the monastery of Saint Peter in Osor and in September 1591 he became canon of the Padua Cathedral.

[1] In Padua he participated to the cultural life, being part and for a certain time also chairman of the Accademia dei Ricovrati, and he became friend of Federico Corner future Cardinal.

The same year he refused the appointment as Latin Bishop of Candia (Crete) to remain in Rome in order to support the negotiations between the Republic of Venice and Pope Clement VIII during the Venetian Interdict.

Agostino Gradenigo was secretly chosen by the Pope as coadjutor bishop with the right of succession to the Patriarch of Aquileia and the Venetian Senate was informed on 27 January 1624.

The nomination as coadjutor was sent secretly to the Nuncio in Venice on 20 March 1627, but was made public only later the death of the previous Patriarch, Antonio Grimani, on 26 January 1628.