He was born in Venice into one of the most ancient, noble and wealthy families of the city, as the son of Gregorio Delfin.

Dolfin was elected while defending Treviso, then besieged by Hungarian troops; being denied safe passage by his enemies, he broke the encirclement and reached Venice to be enthroned.

In the war which had broken out under Dolfin's predecessor, Hungary under King Louis I had conquered Dalmatia and was pushing other Venetian colonies to revolt.

In the resulting Treaty of Zadar the Venetians lost Dalmatia, Zara and Split, but maintained their naval predominance in the Adriatic Sea as the King of Hungary accepted not to build a fleet of his own.

His funerary monument is still visible today, showing the coat of arms of the noble Dolfin family.