In the fourth minute, Ísak Bergmann Jóhanesson became entangled until the end that Samuel Brolin in the Mjällby goal saved - but Maic Sema was able to put in the return.
1-0 and early advantage for IFK Norrköping.
Three minutes later, the guests were close to expanding.
Ari Skúlason pulled from the distance and forced Samuel Brolin to a new save.
This time to the corner.
Norrköping ruled and put in the first half.
They often came close to extending their lead as Ishaq Abdelrazak delivered a low effort on goal from the right after 34 minutes, but the angle was too narrow and the shot went harmlessly into the side netting.
Brolin with a new parade.
Mjällby with final forcing
Mjällby came into the match in the second half, but the away team managed to keep the yellow-clad away.
Skúlason tried a new finish, this time on half volley but the shot was close over the crossbar.
Then Mjällby did everything they could during the final minutes.
And they came close to a draw.
David Löfquist came close to leveling the score 85 minutes into the game, as he was set up with a through ball coming in from the left, but his shot hit the post.
In extra time, Ishaq got a new goal chance, but Mjällby's cage keeper, who made a nice effort today, saved.
The loss is the home team's first at Strandvallen and Norrköping takes its only second away win.
Mjällby will visit IFK Gothenburg next.
Norrköping, on the other hand, takes on BK Häcken.