Strengthened by the victory against IFK Gothenburg, Degerfors started most vigorously at Strandvallen.
Ferhad Ayaz had both chances to give the visitors the lead in the first half.
The minute after his second finish struck just outside the far post, Jacob Bergström got his first position.
Joel Nilsson heel-passed sideways to Bergström during a tangled situation in front of Ismael Diawara in the Dif goal.
But Bergström did not get a proper hit on the ball.
The 26-year-old would have time to take revenge and if Joel Nilsson was disappointed after the first performance was fruitless, he would be all the happier then.
The extremely strong Bergström stayed ahead when Diawara failed to control Nilsson's low finish and poked in 1-0 in the open goal.
Nilsson lifted his heavy teammate in a teddy bear hug.
Bergström would also get the chance to puncture the match.
But almost completely free from half-court, he managed with the piece to roll the ball a good distance outside the goal.
Degerfors was not sharp enough forward to take advantage of it and equalize despite great ball possession in the final stages of the match.
For Mjällby, this means that they took the season's first three points and at the same time pass Degerfors in the table.