When the time came, Dolfin began to serve the Venetian navy, which had the most powerful fleet of its day.
His family had a long tradition in the Navy, reaching special notoriety when Giovanni Dolfin broke the Hungarian siege of Treviso in order to be crowned as Doge in 1356.
In the 15th century, Venice controlled most of the Mediterranean, and had important interests in the rump Byzantine Empire.
Therefore, when the Ottomans began their siege of the Byzantine capital, Constantinople, the Venetian fleet was immediately involved in its defence.
During the siege, Dolfin was commanded to defend the north gate of the city walls, which he managed with great success.