In 2005, at the age of 15, Johnsson moved to Halmstads BK. Johnsson began his career in Ränneslövs GIF. When goalkeeper Marcus Sahlman was loaned to Trelleborgs FF Johnsson was picked up as third choice goalkeeper on the senior team and he made his game debut on 24 August 2008 against Djurgårdens IF when Magnus Bahne was forced to leave the field during halftime. On 20 November 2008, it was reported that Johansson would go toManchester, England, for a week and join Manchester City during training. As Conny Johansson retired and Marcus Sahlman left for Tromsø IL, Karl-Johan was made second choice goalkeeper for the 2009 season. During the 2010 season Johnsson became second choice behind Robin Malmqvist. The 2011 season became turbulent, at first Halmstads BK appointed Josep Clotet Ruiz as new manager of the club, this saw changes among the goalkeepers as Malmqvist was loaned out to Tromsø IL in Norway and Spanish keeper Nauzet Pérez was brought in as first choice, however poor performance and problems with adapting to the Swedish lifestyle saw Pérez depart the club before half the season was played, with his departure Johnsson was appointed first choice as goalkeeper at the club. On 5 November 2012, Halmstads BK announced that Johnsson would leave the club as a bosman for Dutch club NEC Nijmegen when the transfer window opened on 1 January 2013. On 12 July 2019, F.C. Copenhagen confirmed, that they had signed Johnsson on a 4-year contract.
International career
Karl-Johan Johnsson represented Sweden at U19 level before moving up to the Swedish U21 team and debuting against the Slovakian U21 team in 2009. Over time Johnsson was able to work himself into becoming the first choice goalkeeper in the U21 team. Despite Halmstads BK's poor performance in Allsvenskan 2011 and their relegation, Karl-Johan Johnsson was called up to the Swedish national team for the 2012 January tour against Qatar U23 and Bahrain, missing out on the game against Bahrain, he then came on as a substitute at halftime against Qatar, making his senior team debut. In May 2018 he was named in Sweden's 23 man squad for the 2018 FIFA World Cup in Russia.
