Schmidhofer, with just two World Cup podiums in eight seasons, clocked 1min 21.34sec down the Corviglia piste in cold, overcast conditions in St Moritz, touching 60kph in little more than three seconds after leaving the start gate and hitting top speeds of 100kph.
Liechtenstein's Tina Weirather claimed silver at 0.33sec, with Gut claiming bronze — her fifth world medal after three previous silvers and a bronze — a further three-hundredths adrift.
Gut was left licking her wounds somewhat in third, with many backing the Swiss star to have bagged a first world gold.
“It's cool to have a medal, you come to the world champs to win medals,” she said.
“In Schladming (worlds in 2013) I had a medal in the first race, it's a good way to start a world champs at home.
“It's also my first run since my crash last week so I'm just happy I could still ski!”
Gut predicted the course would only improve ahead of Friday's alpine combined and Sunday's blue riband downhill.
“The track is going to get harder for the next races,” she said.
“Next week will be even better.”
The 27-year-old Schmidhofer, whose best previous world performance was fourth in the 2015 Beaver Creek downhill, could barely believe her luck when she streaked through the finish line ahead of Weirather before watching Vonn badly miss her line and ski out halfway down the course.
“I stayed on my feet so that was the most important thing,” said Vonn, coming back from a broken arm that leaves her hand without total grip.
“I'm definitely disappointed about today. I lost my pole and lost my concentration for a minute.
“I tried to go too straight to make up for the time I'd lost — that was a bad tactic as I went over the roll and had no chance to make the gate.”
Vonn admitted that the lack of grip in her hand meant for drastic measures ahead of Sunday's downhill.
“Going forward, I'm just going to have to duct-tape my hand to my pole because I can't afford to lose it again,” she said, playing down any concerns that she had been nervous.
“I felt good and I felt confident in the start. I wasn't nervous. I was ready and I attacked.
“Sometimes it just doesn't work, that's ski racing and I can't let it get me down.
“I just have to keep focusing on tomorrow and try to build my confidence and try to get the timings for the downhill right.
“I think I'll be in good shape for Sunday.”
After Vonn's departure, threats still remained for Schmidhofer in the form of Germany's giant slalom specialist Viktoria Rebensburg and Austrian Anna Veith, the reigning Olympic and world champion who missed last season through injury.
Rebensburg eventually came fourth, at 0.53sec, while Veith bombed out after badly negotiating a cut-up part of the slope, leaving Schmidhofer to breathe a huge sigh of relief in the finish area.