After a degree in Spanish language and literature, Giornetti attended the fashion design course at Polimoda in Florence in collaboration with the London College of Fashion and the Fashion Institute of Technology of New York. Giornetti’s career in the fashion industry started in Rome where he collaborated with the designer Anton Giulio Grande for the development of the RTW and Haute Couture collections. 
In July 2000, he joined the men’s division at Salvatore Ferragamo, becoming creative director in 2004.
In January 2010, he began designing the women’s ready-to-wear collection and after a few months he was appointed creative director for the entire Ferragamo label, becoming the first creative director with the task to coordinate and orchestrate all the categories of products, PR, events and merchandising.
In 2011, Giornetti presented the first Salvatore Ferragamo women’s resort collection with an intimate show held in the former residence of heiress, philanthropist and art collector, Doris Duke.
In 2012 Salvatore Ferragamo was involved in a project combining art and history: the Florentine company sponsored the ‘Saint Anne - Leonardo Da Vinci’s Ultimate Masterpiece’ exhibition at the Louvre between 29 March and 25 June 2012. To mark the occasion, Giornetti presented the Salvatore Ferragamo women’s resort 2013 collection in the Denon Wing of the Louvre, marking the first time that the museum had opened its historic internal spaces for a fashion show.
In October 2013, at the opening night of Wallis Annenberg Center for the Performing Arts in Beverly Hills, Giornetti designed a Haute Couture capsule collection dedicated to the founder of the Florentine Maison Salvatore Ferragamo and his Hollywood debut.
From 2017 to 2019, Giornetti collaborated on the rebirth of the iconic Chinese luxury brand Shanghai Tang, redefining the aesthetic and values of the house.ù
In October 2019, Giornetti joined Polimoda as Head of Design Department.
In February 2021, Giornetti was appointed Director of Polimoda.