Born in Höganäs, Johnsson began his footballing career with Öresunds BK before moving on to represent Utbynäs SK, Vårgårda IK, Höganäs BK, and Redbergslids IK until signing for the Allsvenskan club GAIS in 1948. While at GAIS, Johnsson helped the team finish third in the 1948–49 Allsvenskan by scoring 14 goals in 21 games. The following season, Johnsson scored 9 goals in 22 games as GAIS finished 5th. It was at GAIS that Johnsson earned the nickname "Atom-Egon," referring to his strong physique and powerful shooting skills.
