Leonardi was born in the Venetian city of Traú (today's Trogir, in Croatia), in Dalmatia.
Leonardi began as an episcopal vicar, however, due to his education and abilities, he became a delegate of the Congregation of Religious Propaganda under the Roman Curia.
In 1638, Leonardi met with Mardarius, the Orthodox Metropolitan of Cetinje, to discuss his potential conversion to Catholicism.
Mardarius expressed his readiness in accepting the offer, also demanding that the Roman Curia allot the same wages as the bishops under the Ottoman Empire.
Upon his release from prison in early 1640, he met with Leonardi in Maine and converted to Catholicism, acknowledging submission to the Pope.