Giovanni Giustiniani was either a Genoese official or Pirate (there were a lot of Giustiniani around) who raised a company of about 700 men in January 153 the service of Emperor Constantine XI.

Constantine XI entrusted Giovanni Giustiniani with "the most critical point on the walls" around the Gate of St. Romanus (right in the middle of the walls) and his company was possibly augmented by members of the large Genoese community in Constantinople.

In a notable incident during Medhmet II's final attack on the walls in April 1453, Giovanni Giustiniani pointed his sword against the Byzantine commander in chief, Megas Doux Loukas Notaras, because he was unable to supply the promised gunpowder for the cannons. The Megas Doux had previously said he would rather be under the yoke of the Ottomans than the "Latins" (Western Europeans) and Giustiniani accused him of treachery.

As the situation became more desperate, Giovanni Giustiniani came to command all the Latins fighting for the Byzantines, until he was mortally wounded by a Janissary. Carried to a Genoese ship he and what remained of his company sailed for the Genoese outpost on the island of Chios, where Giustiniani died of his wounds.

Although he was a charismatic leader (Emperor Constantine seemed to take a linking to him) the Byzantines were outnumbered 11-1, and in the hopeless situation there was little impact Giovanni Giustiniani could have.