Hirscher was chasing a third gold medal in Pyeongchang – to add to his combined and giant slalom titles – but the Austrian lost balance midway through his run to blow the race wide open.
Kristoffersen had gone out just before Hirscher, and the giant slalom silver medallist was ultimately ahead at the halfway point after crossing the line in 47.72 seconds.
Swiss oust great rivals Austria to win inaugural Alpine team event gold
However, Kristoffersen fluffed his lines in the second run, with Myrher going on to take gold, 0.34secs ahead of Switzerland’s Ramon Zenhaeusern and 0.67secs clear of Austria’s Michael Matt.
Meanwhile, GB's Dave Ryding secured a top-10 spot when coming ninth in his third Olympic Games, with the 31-year-old finishing 1.17 seconds behind Myrher.
Kristoffersen’s nearest challenger after the first run was Myhrer (0.21secs behind), while Ryding was down in 13th – 1.37secs off the pace.
Quite staggeringly, just 52 of the 106 starters completed the first slalom run, though it was the DNF next to Hirscher’s name which stood out greater than most.
"For sure it is disappointing," the six-times overall World Cup champion told reporters. "But I already had a bad feeling about the whole situation. It was kind of s****y already before the race started.
"I couldn't find any speed. We were not able to find something that makes it possible to ski around the gates smooth and clean... if it is grippy and hard, packed snow like here today, I'm really not able to handle this.
"This is as well the mistake of my technician and my powerful skiing -- the smoother you ski, the better you are in these conditions."
Going into the second run without Hirscher’s domineering presence, Kristoffersen was hoping to upgrade the bronze he achieved in Sochi to gold at the Yongpyong Alpine Centre.
But after watching Myher cross the line with a combined time of 1:38.99, Kristoffersen could not see out the victory, giving the Swede a shock win - eight years on from his bronze in the same event at the Vancouver Games.