Sinibaldo Fieschi, his father, had been a close friend of Andrea Doria, and had rendered many service to the Genoese republic. On his death in 1532, Giovanni found himself at the age of nine the head of the family and possessor of immense estates. He was described as handsome, intelligent, of attractive manners and very ambitious. He married Eleonora Cybo, marchioness of Massa, in 1540, described as a woman of great beauty and influence.
A number of armed men from the Fieschi fiefs had been secretly brought to Genoa, and they agreed that on the January 2 1547, during the interregnum before the election of the new doge, they should seize the galleys in the port and the city gates.
The first part of the plan was easily carried out, and Giannettino Doria, aroused by the tumult, was killed as he rushed to the port. Andrea Doria, however, fled from the city unharmed. The conspirators attempted to gain possession of the government, but unfortunately, Giovanni Luigi, while crossing a plank from the quay to one of the galleys, fell into the water and drowned. The news of his death spread consternation among the Fieschi faction, and Girolamo Fieschi found few adherents. They came to terms with the senate and were granted a general amnesty. However, when Doria returned to Genoa on 4 January, he ignored the amnesty, and confiscated the Fieschi estates. Girolamo, Verrina, Sacco and other conspirators had secluded themselves in his castle of Montobbia, which the Genoese, at Doria's instigation, besieged and captured. Girolamo Fieschi and Verrina were tried, tortured and executed; all their estates were seized, including Torriglia, which Doria obtained for himself. Ottobuono Fieschi, who escaped, was captured eight years later and put to death by Doria's orders.