Designated as Governor of Procida (an island off the coast of Sicily) by the Hohenstaufens, Gianni entered the service of Frederick III as a medical doctor, and later upheld the claims of Manfred and Conradin to Sicily. After the defeat of Conradin at Tagliacozzo in 1268, Gianni da Procida joined the Aragonese court, where he played a significant role in the conflict between Charles I of Anjou and Peter III for control of Italy. Popular opinion held that Gianni had been instrumental in instigating the revolt of the Sicilian Vespers. The Florentine chronicler Giovanni Villani portrays him as a "savio e ingegnoso cavaliere e signore" stating that "Questi per suo senno e industria si pensò di recare la forza del re Carlo in basso stato, e in parte gli venne fatto" (VIII.57).

Boccaccio includes Gianni da Procida in two tales (II.6 and V.6); in both he is described in laudatory terms as honest, courageous, and loving. Boccaccio's positive view of Gianni, despite his Angevin sympathies, is also demonstrated in the De Casibus IX.19.

(R.P./N.S.) Villani, Giovanni. Nuova Cronica, 3 vols, ed. Giuseppe Porta. Parma: Guanda, 1990.