Mjallby were riding high after ending their long winless run in the league by defeating Sirius, who were previously unbeaten and cruising towards the title, but taking on a Red Bull Salzburg side with a proud European pedigree would be a real challenge for the continental newcomers.
Edmund Baidoo gave Robin Wallinder a simple save to make five minutes in, but that set the tone for a first half the visitors dominated. Yorbe Vertessen gave Wallinder more catching practice before the goalkeeper got down low to deny Damir Redzic who cut in well from the right.
Dominik Schmid then became the focal point of the Salzburg attack, as he received a square ball inside the area and unleashed a low effort that Wallinder parried with his legs, and less than a minute later, a cross into the box was half cleared to Schmid at the edge of the box, and his clean first-time hit was palmed away.
Late in the half, Redzic controlled a long ball well and left his defender in his wake, but couldn't beat Wallinder from a tight angle and Bartosz Mazurek's effort on the rebound was blocked, allowing Mjallby to keep the game goalless at half-time.
Had Max Nielsen kept his composure and not fired over when the ball fell to him three minutes into the second half, MAIF could have taken an unlikely lead, before Jeppe Kjaer and Olle Lindberg both fired wide in quick succession, as the home side started the second half the brighter.
The visitors looked to be getting back on top when a Schmid corner was met by Mazurek and headed off the line, only for Mjallby to go close from a corner of their own at the hour mark - Aki Samuelsson's delivery was headed onto the crossbar by Pettersson.
Both sides produced little more as the half wore on, until a beautifully floated free-kick from Kjaer was headed back across goal by Pettersson and met at the other post by Tony Miettinen, who stretched every sinew but couldn't direct his header inside the upright.
The Mjallby defence had been forced to dig deep, putting in a series of blocks to keep the scores level, but in added time, their resistance finally broke when Kamir Konate got to the byline down the left side, and his ball across the face of goal was put into his own net by a scrambling Pettersson, under pressure from Sota Kitano.
There was almost no time for Karl Marius Aksum's side to respond, and now the two sides will meet again in Austria on Thursday 27th, knowing UEL League Phase football awaits Salzburg if they can avoid defeat, and as things stand Mjallby will be entering the League Phase of the UEFA Conference League.