While all eyes were on Vonn who was making her comeback following a three-week break, McKennis stole the show in one minute and 14.62 seconds.
The Colorado skier, 23, was the fourth American downhill specialist on a women's World Cup podium this season after Vonn, Stacey Cook and Leanne Smith while youngster Mikaela Shiffrin emerged as the new slalom sensation this winter.
"It hasn't sunk in yet. I cannot really understand what's happening. It's a total surprise. I never thought I could win such a difficult downhill, I was aiming at a top 10 finish," a jubilant McKennis said.
McKennis made the best of the peculiar conditions - the start was brought down to the Super-G start after fresh snow fell overnight, forcing organisers to work hard on the piste for the race to go ahead.
The American's previous best World Cup result had been seventh in Schladming last season.
"The key was probably the early start number and the smooth course. Still this is totally beyond my imagination," she added.
In the finish area, she was cheered on by Vonn, back from taking time off to deal with health and personal problems.
The four-times World Cup champion, who made official her divorce from Thomas Vonn this week, had to be content with sixth place, 0.34 seconds behind her compatriot.
"The break did me a lot of good. It had been 11 years since I hadn't spent New Year’s Eve at home. This was just a return race. I'm going to improve with each race," she said.
"I'm really happy for Alice as she has had lots of injuries in the past. I think everybody in the team has been on the podium this season which is great."
Italy's Daniela Merighetti was second, 0.07 seconds adrift while Austria's Anna Fenninger, fresh from a giant slalom victory in Semmering, was 0.16 off the pace and finished third.
World Cup leader Tina Maze of Slovenia was a solid fourth, 0.30 behind, and took her overall lead to 488 points over Germany's Maria Hoefl-Riesch.
"It's a shame not to be on the podium but it's also good to know I can do well in a downhill. I left the incident of straddling a gate in Zagreb behind me and I'm sticking to my plan to rake each race as it comes and see where we are in March," she said.
Sunday's Super-G on the same Karll Schranz course will be another important goal for the Slovenian, as it is the only discipline in which she has yet to win a World Cup race.
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