Watch Mjällby's victory dance in the player above.
The 30 or so travelling fans were thanked by Mjällby's team and could celebrate while the home fans booed their players.
Norrköping were on for an equaliser when Mjällby came in a quick changeover. Despite having to stop on the counter-attack, the ball went to Leo Walta who from his left wing drifted inside and made it 2-0 with five minutes left of regular time.
See all highlights from Allsvenskan in Play.
CLIP: Mjällby's goal in the win against Norrköping
Javascript is disabled
Javascript must be turned on to play video
Learn more about browser support
Mjällby zeroed in on Beijing in Norrköping