Serge Gennaux had his first publications in Héroic Albums ('Du Grisbi en Vrac') before becoming the apprentice of Maurice Tillieux in 1957. Two years later, he joined the lettering studios of the publishing house Dupuis. Starting in 1960, he was one of the most prominent providers of short filler comics for Spirou magazine, which he made either alone or in cooperation with Charles Degotte, Hubuc or Salvé. Between 1968 and 1972, he drew his first series, starring the gangsters 'Loryfland et Chifmol', with scripts by Raoul Cauvin. In addition, Gennaux wrote scripts for Willy Lambil's series 'Hobby et Koala' and 'Sandy et Hoppy'. During this period, he also cooperated on the first animated Smurfs cartoons.
In 1965, Gennaux created his best-known character, 'L'Homme aux Phylactères'. Starring in series of gags about a man and his word balloons, the strip played with the conventions of the comics medium and after some first appearances in 1965-66 it became a regular feature in Spirou until between 1978 and 1985. In addition, Gennaux was present in Tintin with the humorous medieval series 'Le Chevallier qui Venait du Froid'. In 1970, he began a collaboration with the artist Jamic to create the TV parodies 'Les Télé-Graphistes' and 'Les Émissions Impossibles' for Télé-Moustique. In later years, Gennaux dropped most of his comics activities and was employed in the graphic departments of the magazines Humo and Télé-Moustique.