He was the son of Ricciarda Malaspina, duchess of Massa and Carrara and Lorenzo Cybo, count of Ferentillo who changed his name to Cybo-Malaspina, and a nephew of Pope Innocent VIII. In 1546, he challenged his mother for control of the duchy of Massa and Carrara, which she had inherited from her father.
With the backing of Cosimo I de" Medici and Andrea Doria, he seized control by force in 1546. Soon after, his mother appealed to Charles V, Holy Roman Emperor, and Charles sent his forces to seize Massa while Guilio was away. Guilio had been promised a large dowry, which he wanted to fund a return to power, but Andrea Doria refused, arguing that he"d already spent more than the dowry on Guilio"s first attempt to seize the Duchy.
When Doria refused, Cybo entered a conspiracy to oust the Admiral from the rule in Genoa, and replace him with a French protectorate.
With the help of the pope and Piero Strozzi, the revolt should spread to the whole Italy, with the objective to expel the Spaniards from the country. The plot was discovered before its beginning and Cybo was arrested in Pontremoli.
Despite the intervention of Cosimo I de" Medici in his favour, he was executed in May 1548.
Guilio was to help Ottobuono Fieschi and other Genoese exiled in Venice to enter the city and kill Doria, the Spanish ambassador and other members of the Doria party.
