Jansson began his career with BK Häcken, starting in their youth academy and making his professional debut on 19 May 2010 in a Svenska Cupen third round game against Östersunds FK, his only appearance of the season.

He went on loan to Division 1 Södra side FC Trollhättan for the 2011 season before leaving Häcken at the end of the year.

[5][6] Having joined from the Swedish third division as emergency injury cover, Jansson was a surprise success at AIK, featuring as a regular starter in one of the league's best defensive teams.

On 11 November 2018, he scored the decisive goal in a 1–0 win over Kalmar FF as the team secured the Allsvenskan title for the first time in nine years.

[11][12] Having never played for Sweden at youth level, Jansson's league-winning form with AIK caught the eye of Janne Andersson who called him up in December 2018.