He fought at sea against the Ottoman Sultan Mehmed II and on land against Ercole I d'Este, duke of Ferrara, from whom he recaptured Rovigo and the Polesine.

Giovanni Mocenigo was born in Venice in 1408, to a very distinguished family: both his father, Leonardo, and his grandfather, Pietro, were appointed Procurator of Saint Mark.

In November 1469 he was elected an avogador di Comùn, but held the office for a few weeks before being appointed luogotenente (governor) of the Patria del Friuli, serving until August 1471.

[3] In 1474 he was a savio del consiglio, before being sent, along with Andrea Vendramin and Antonio Venier, to negotiate the extension of the Peace of Lodi with Florence and the Duchy of Milan.

His election was in large part to his popularity for his upright character rather than any political skills, and was won with crucial support of Marco Corner, the brother-in-law of Mocenigo's brother Nicolò.

[3] The long war with the Ottomans, going on since 1463, was concluded in January 1479 with the Treaty of Constantinople in a clear Venetian defeat, as Venice had to recognize the loss of Negroponte, Scutari, and other fortresses.