Injured Aksel Lund Svindal, hoping to recover in time from a torn Achilles tendon to compete at next month's world championships, was among the first to congratulate Jansrud, embracing him in the finish area.
"It was so good to see him," Jansrud said, before joking "but it's always dangerous when you see him smiling and laughing because that means his Achilles is probably pretty good and he will probably be faster than us at the worlds."
With his third downhill victory of the season, Jansrud became the first Norwegian winner on the Streif since Lasse Kjus 11 years ago.
Svindal, who has been racing here since 2003, had his best result when he placed second last year.
"I am really happy that Kjetil won," Svindal said. "Watching a race from the sideline is not where you want to be. But that's life."
With 45,000 spectators waiting in light snowfall, fog on the upper part of the course forced organizers to delay the race by two hours, and then significantly lower the start gate as conditions failed to improve.
The clouds limited visibility, making it potentially dangerous for skiers who reach speeds of up to 140 kph (90 mph).
In the shortest downhill in World Cup history, Jansrud clocked 58.16 seconds — half the time usually needed on the 3.3-kilometre (2-mile) course — to edge Dominik Paris by 0.02. The Italian won the super-G on the same course Friday.
Guillermo Fayed of France came 0.21 behind in third for his second career podium.
"'Kitzbuehel champion' sounds really cool," Jansrud said. "It's an amazing day, a huge victory for me ... Winning here was a big goal of mine."
Benjamin Thomsen of Invermere, B.C., was 17th.
While topping the leaderboard, Jansrud saw Paris clocking faster split times but the Italian lost more than three-tenths in the finish section.
"I wasn't happy that we started from the Seidlalm as my best part is the upper part of the hill," Jansrud said. "We always want to run the full race in Kitzbuehel. But first priority is the safety so it was a good call."
Jansrud closed the gap on overall World Cup leader Marcel Hirscher, who doesn't compete in downhill, to 82 points. However, the Austrian has two slaloms coming up — in Kitzbuehel on Sunday and nearby Schladming on Tuesday — to extend his lead.
Paris came just short of his second win in two days.
"Those two-hundredths annoy me a bit," said the Italian, who won the downhill two years ago. "But it's great to be on the podium. The race was great. Even with the shorter run, it was a fair race."
American Steven Nyman, who won the downhill in Val Gardena last month, was 0.41 back in fifth, and Olympic champion Matthias Mayer of Austria trailed Jansrud by 0.47 in 10th.
Last year's winner Hannes Reichelt of Austria finished 1.23 off the lead and shared 34th with Swiss skiers Mauro Caviezel and 2009 winner and 2010 Olympic champion, Didier Defago, who raced the Streif for the last time before retiring at the end of the season.
It was the final men's World Cup speed race before the world championships in Beaver Creek, Colorado, in February.
The 75th edition of the traditional Hahnenkamm races is concluded by a slalom on Sunday.