At his election in 836 he nominated his son Giovanni as co-regent, continuing the process begun a century earlier of establishing a hereditary dukedom with dynastic succession.

He led a large fleet against in 839 to punish the Narentines (also called Neretvians) for raiding and slaughtering several Venetian traders returning from Benevento in 834–835.

He made peace with Duke Mislav of the Croats of Dalmatia and a Narentine prince by the name of Družak (Drosaico, Marianorum judice).

In Spring 856, Pietro and his son and co-regent Giovanni hosted Emperor Louis II and his wife Engelberga in a three-day visit to Venice.

Pietro himself died a year later on 13 September 864, having been ambushed and attacked by a group of malcontents while returning from vespers at the Church of San Zaccaria.