Jansrud, who had not performed well during Friday's super-G, recorded a time of 58.16 seconds, two-hundredths of a second ahead of the Italian Dominik Paris.
The time was half of what would usually be expected to win the title.
Appalling weather conditions, including heavy fog and snow, severally curtailed the race. Such was the poor visibility that the organizers moved the start to the Seidelalm jump, half way down the two mile course.
It meant that many of the course's most terrifying features, including the opening Mausefalle -- or mousetrap -- which sees competitors reach speeds of close to 100 mph, had to be bypassed.
'Tiger turned into a kitten'
Jansrud had predicted that the course would be easier than usual.
In a tweet sent before the race he said that the course was now a " tiger turned into a kitten, but still: Safety first."
But that didn't detract from what must rank as the highlight of Jansrud's career so far.
"Winning in Kitzbuehel has been a goal for many years," the 29-year-old said after his victory.
"To make it happen is pretty extraordinary."
Vonn flops, finally
In the women's downhill in St Moritz Lindsey Vonn finally proved she was fallible after a rare mistake saw her finish 23rd.
Vonn, who wore the slogan "I'm the Best" on her bib, had the chance to extend her World Cup win record.
Instead Switzerland's Lara Gut gave the home crowd something to cheer with a winning time of 1 minute 43.82 seconds.
"I simply looked to find the shortest line," Gut said after her victory.
"It was very tough to ski."