The son of an engineer and a painter, he knew he wanted to become a comics artist since he was ten years old. He learned the finer points of the profession from Vittorio Leonardo, taking courses at the Academy of Châtelet.
He subsequently studied at the School of Graphic Research in Brussels, focusing on animation. In the evenings, he worked with Eddy Paape, who introduced him to the publishing house Lombard in 1984. In 1984-85, he drew the humorous series 'Aldose et Glucose' in the publisher's magazine Tintin. A few years later, he made his first realistic comics from scripts by the Rahir sisters for the same magazine.
In 1989 he joined the publishing house Claude Lefrancq and began working on the fantasy detective series 'Carland Cross' with writer Michel Oleffe.
In 1999, Grenson teamed up with Jean Dufaux to create the thriller series 'Niklos Koda' for the collection Troisème Vague of Lombard.
In addition to drawing this series, he created 'La Femme Accident' for the Aire Libre collection of the publishing house Dupuis in 2008.
He became an allround artist with 'La douceur de l'enfer', a diptych published by Le Lombard in the collection Signé in 2011 and 2012.