Mjallby were protecting an eight-match unbeaten run in the league as they welcomed an inconsistent AIK to Hallevik, knowing there would only be two points separating them should the visitors end that run.
The first half had few chances and by far the best fell to the hosts' Alexander Johansson, who ran onto a ball over the top, flicked it over the defender's head and cannoned it against the cross bar.
10 minutes after the break, the leaders found a way through thanks to a touch of fortune - a free-kick from the right from Elliot Stroud was heading wide before it deflected off Johan Hove standing in the wall and sailed into the back of the net.
Mjallby hit the woodwork again and coughed up big chances before finally sealing a deserved three points seven minutes from time - a ball into the box wasn't dealt with by the AIK back line and Ludvig Tidstrand pounced and fired into the roof of the net.
The win takes Mjallby to 40 points, eight clear of AIK who were defeated for the third time in their last five league matches.
Mjallby's weekend looked to be getting even better later on Sunday, when Brommapojkarna went 2-0 up against Hammarby.
Both BP goals arrived late in the first half courtesy of Adam Jakobsen, who swept home Victor Lind's free-kick in the 43rd minute, before bursting into the area and holding off challenges to score his second in added time.
But after four substitutions at the break, Hammarby came roaring back in the second half, cutting the advantage within nine minutes of the restart when BP failed to clear a cross and Nahir Besara was on hand to nod home.
Pressure on the visitors was telling, as a Bajen corner in the 72nd minute was bundled into his own net by Kevin Ackermann to level the scores. Four minutes later, the turnaround was complete - a blocked shot fell kindly for Montader Madjed, who fired home the winning goal.
The crucial 3-2 win keeps Hammarby within four points of Mjallby, while BP saw their three-match winning run end in dramatic fashion, and they drop to 10th.
Malmo are now breathing down AIK's necks as they went five league matches unbeaten by beating Oster 2-0 away from home, now sitting two points behind the Solna side.
Otto Rosengren netted the opener midway through the first half, before setting up the clincher midway through the second for Sead Haksabanovic. Oster remain in the relegation zone after three losses running.
The champions have overtaken Elfsborg in the standings, while Djurgarden were the team to overtake BP, as August Priske's early goal was enough for the Iron Stoves to win 1-0 at home.
One goal was all Goteborg needed, too, as they beat Sirius for the second time this month. David Kruse's strike just before the hour mark was the difference maker, as Goteborg remain seventh and Sirius slip to a third defeat in a row.
However, the team from Uppsala are staying above the bottom three thanks to Degerfors, who were beaten for a seventh match in a row, this time 3-0 at home to GAIS.
Matteo de Brienne and Ibrahim Diabate were on the score sheet for the visitors, either side of a Juhani Pikkarainen own goal, taking GAIS to sixth place and their ninth match unbeaten.
Halmstad and Hacken sit 12th and 8th, respectively, after the two played out a goalless draw, extending the visitors' unbeaten run to three games.
Round 16 was completed on Monday evening as Varnamo failed to build on their first win of the season by falling to their 11th defeat, 3-1 away to Norrkoping.
Christoffer Nyman took less than three minutes to open the scoring for the hosts, but that was cancelled out early in the second half by Hugo Andersson.
But no sooner had Varnamo got themselves later did they concede twice in the next seven minutes - Arnor Traustason put Norrkoping back in front before Nyman notched his second to secure a first win since mid-May.
