With Hammarby losing for a second week running and only one of the teams lying second to seventh picking up a win this weekend, Mjallby were desperate to take full advantage having only managed a draw last week.
But with only one loss in their previous 13, GAIS away would provide one of the toughest tests the Allsvenskan could throw up.
That seemed to be the case in the first half in which neither side could force a breakthrough - Amin Boudri forced a good save from Noel Tornqvist in the Mjallby goal, while at the other end, Jeppe Kjaer planted a header narrowly wide.
The league leaders were still under the cosh early in the second half; Tornqvist made an excellent save to deny Jonas Lindberg, before tipping a Kevin Holmen effort around the post.
But just after the hour mark, the Hallevik side took the lead against the run of play - Herman Johansson beat goalkeeper Kees Sims to a long ball over the top and knocked it down into the path of Abdoulie Manneh, who took advantage of an out-of-position Sims to fire in the opener.
GAIS came closest to an equaliser through a Shalom Ekong effort in the final 10 minutes, but with 87 minutes on the clock, Mjallby secured the points when a deep Viktor Gustafsson corner was nodded in at the back post by Johansson to make it 2-0.
After their long unbeaten run, it's two defeats in three for the Gothenburg side, yet they remain two points off the European places. Mjallby have hit the 50-point mark and now gone 13 league matches unbeaten.
Monday's other match saw Norrkoping put further distance between themselves and the bottom three with a 2-1 win away at Oster, who remain in the relegation play-off position.
The visitors had been knocking on the door throughout the first half and took a deserved lead in the 37th minute in fine style - David Moberg Karlsson cut in from the left wing and curled a delightful effort into the top corner.
Eight minutes after the break, Karlsson scored his second of the night when timing his run perfectly to latch onto a through ball from Isak Sigurgeirsson and steer his shot past the goalkeeper.
Oster finally began to create having gone 2-0 down, and got themselves a goal back eight minutes from time when Magnus Christensen forced the ball over the line amid a goalmouth scramble, despite the best efforts of the Norrkoping defence.
Seven points and four places separate the two sides after back-to-back wins for Norrkoping, while Oster's run of four matches without defeat came to an end.
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