That lead could be cut back to eight points depending on how today's Swedish top flight matches go, but Mjallby have done all they can to put pressure on their nearest challengers, Hammarby and AIK.
The first half had few chances, with the best a rocket of a strike from Viktor Gustafson which was beaten away by the goalkeeper following a short free-kick.
Halmstad had opportunities of their own after the break to throw a cat among the pigeons, but 10 minutes from time it was the home side who finally broke the deadlock.
Ludvig Tidstrand was clipped from behind inside the area, and Elliot Stroud converted the penalty in off the post to earn MAIF another three points and extend their unbeaten run to 14 matches.
Halmstad can be proud of their performance, but a third loss in four keeps them only three points clear of the relegation zone with their two nearest challengers in action today.
Having slipped out of the title race, Elfsborg are now falling away in the race for the European places after they suffered a third defeat on the bounce, this time 3-0 away at Brommapojkarna.
The match was somehow still goalless at the break as the visitors endured endless pressure from BP and had goalkeeper Simon Eriksson to thank for a string of vital saves.
But 10 minutes after the break, the home side finally took a deserved lead when a superb Oliver Zanden pass from wide on the left found Victor Lind, who poked it past Eriksson despite the tight angle.
Midway through the second half, Lind was afforded too much space on the right side, strolled into the penalty area and smashed the ball into the top corner to score his and the Bromma Boys' second.
Seven minutes from time, a ball over the top found Alex Timossi and he too beat Eriksson from the right of the penalty area, only he drilled a powerful effort low at the neat post which the goalkeeper will feel he should have done better with.
Elfsborg now sit sixth and could drop as low as eighth if other results fo against them. BP's first win in four matches moves them up to ninth for the time being.
Elfsborg have dropped a place thanks to IFK Goteborg moving into third place after Saturday's other match, in which they beat Varnamo 1-0 at home.
Against the division's bottom side, it wasn't a vintage performance for Goteborg, but they increased their unbeaten run to five matches and took all three points with a goal on the stroke of half-time.
A corner ended up at the feet of David Kruse at the back post, and he sent it back across goal for Noah Tolf to tap in from close range.
Having wasted many chances to extend their lead, IFK had to hang on desperately in the final minutes, but a lack of cutting edge was Varnamo's downfall and a second successive loss sees them stay six points behind the relegation playoff place.