Like most of his Genoese colleagues in Antwerp, Giovanni Giacomo Fiesco belonged to the nobility. His noble descent is mentioned in a request he made in January 1578 to the Antwerp aldermen, to leave the city. To increase his chances to a positive answer, the heading of the letter was changed appropriately ("Fiesco marchant" was crossed out and replaced by "Fiesco gentilhomme") and three friends witnessed his reputation as a respectable nobleman:

[Transcription] "A la requeste de Jan Jacomo Fiesco (marchant) [crossed out] gentilhomme [inserted] Genevois traffiquant et resident en ceste ville danvers... Adam Clermont ... Thirry vanden Hove ... et [?] Rubbens ... iuraverunt ... quilz scaivent de vray que icelluy Fiesco est gentilhomme de bien de bonne fame et renommé sans avoir oncques entendu ... le contraire." [Translation] "To the request of Jan Jacomo Fiesco (merchant) [crossed out] gentleman [inserted] from Genoa trading and residing in this city of Antwerp ... Adam Clermont merchant ... Thirry vanden Hove ... and Rubbens [?] ... swear ... that they know for sure that this Fiesco is a gentleman of irreputable fame and that they have never heard the contrary."

Antwerp, City Archive, SR 354, fol. 103v

Being in the comfortable position as merchant ànd nobleman Fiesco displayed a keen interest in culture. He loved tapestry, wrote poetry, was a friend of the author Guillaume De Poetou, and financially supporting the latter's debut. He probably played music himself, as is suggested by one of the poems of De Poetou. Moreover, we know he owned at least one small organ.

However, his role as a patron and his love of music are most manifest in his support of Jean De Castro's debut. Undoubtedly Fiesco met the young composer in Antwerp, took an interest in his music and decided to encourage him to publish his first book. He presumably supported him financially in the publication (as Stefano Gentile had done for Lassus) and also seems to have been the author of one of the madrigal texts in this debut:

Contrary to all the other madrigals of the volume, the author of "Chiara luce" and the context in which it is written are unclear. As far as is known, this text has never been used by another composer. In view of Fiesco's poetical ambitions it becomes an attractive theory that Fiesco himself has written the text of the madrigal.