He began his comics career in 1953 as an assistant to Nils Egerbrandt on the comic strip "Olli". During the early 1960's, the Swedish Fantomen comic book was in need of new material, and obtained the rights to produce original "Phantom" stories. Wilhelmsson was selected to be the first artist. This was the beginning of what is now referred to as Team Fantomen. Wilhelmsson is usually credited as writer for these early stories, however it is possible that some were written by others, such as Ebbe Zetterstad.
In the 1960's Wilhelmsson also worked on the strips "Uncas" and the Swedish comic books stories of Hank Ketcham's "Dennis the Menace".
Around 1965 Wilhelmsson and his family moved to Malaga, Spain. At the end of the 1970s he moved back to Sweden to settle down in the Stockholm area.
Wilhelmsson took a break from "The Phantom" from 1978 to 1986, and mostly worked on comedy strips such as "Flygsoldat 113 Bom", "91:an", "Frisk och Rask" and "Buster".
In 1976–1979 and 1986–1988 he frequently collaborated with Özcan Eralp.