Born in Rome, Guglielmo Letteri has lived in several countries, including Albania and Brazil. He did several jobs before beginning his professional comics career, like playing in a jazz band. From 1948, he worked for the Argentine Abril and Columba publishing houses, along with artists and writers like Hugo Pratt, Alberto Ongaro, Sergio Tarquinio and Mario Faustinelli. Moving several times throughout the 1950s, he also took on comics work for the British Fleetway agency. Back in Italy in 1963, he became one of the main artists of 'Tex Willer' (originally created by Gian Luigi Bonelli and Aurelio Galleppini) for Bonelli publishers.