In Vonn's first start since setting the all-time World Cup wins record this week, her race unraveled when she hit a rut midway down the new Engiadina course while trailing 0.05 seconds behind Gut's time split.
Vonn was pushed wide into soft snow and lost all speed before finishing more than two seconds back in the final downhill before the Alpine world championships next month at Vail-Beaver Creek, Colorado, where she lives.
"It's disappointing but at the same time I skied well, just that one mistake," Vonn, who placed 23rd, told The Associated Press.
Gut raced down in 1 minute, 43.82 seconds to get the downhill victory here that home fans had wanted since, at age 16, she first made a World Cup impact as a third-place finisher in 2008.
"It's cool for me to finally find a way back," said Gut, who likely would have won seven years ago but for crashing and tumbling sideways across the finish line. "Of course, it's a special feeling here."
Anna Fenninger of Austria was 0.32 behind Gut, and now has six runner-up finishes and no victories in defence of her overall World Cup title.
By placing third, Romania-born Edit Miklos got a first podium finish for her adopted country Hungary in 48 years of World Cup racing. She finished 0.58 behind Gut, taking advantage of her No. 2 bib to race down in lighter winds on a clear, sunny day.
Larisa Yurkiw of Owen Sound, Ont., was the top Canadian in 19th.
Race conditions seemed ideal for Vonn to extend her record of 63 career World Cup wins, and organizers played a dance track with the lyrics "You're a superstar" in the finish area when she started.
Vonn was fastest through upper sections but hit trouble at a tight left-hand turn and was almost turned around in the course-side snow.
"I was at a funny angle and my inside ski kind of booted out," she said. "I did the best I could to recover it but clearly stopped."
Vonn still leads the downhill standings though Fenninger used her 80 race points Saturday to close the gap to 79. Two races remain in March.
A super-G is scheduled for Sunday, the same discipline Vonn won on Monday at Cortina d'Ampezzo, Italy.
"I'm confident and feel good going into world championships, no matter what happens tomorrow," the 30-year-old American said of the last race before the Feb. 2-15 event.
Gut's second win this season, and 12th of her career, sets her up for the worlds on a steeper and more technically demanding Beaver Creek course which she prefers.
"I only raced it once and I was pretty fast," Gut said. "People said, 'St. Moritz and you is a perfect match' but I had only won once."
The new downhill layout at St. Moritz was raced for the first time in World Cup on Saturday and will host the two-yearly worlds in 2017.
"I think we have pretty exciting world championships to come," said Gut, who has three worlds silver medals but no gold.
Fenninger, the Olympic champion in super-G, has specialized in second places this season.
"I don't know why every time it is one girl faster," said the Austrian, who has been runner-up to six different rivals. "In December I had some problem with self-confidence but now it's 100 per cent back."
Fenninger is also second in the overall standings to Tina Maze of Slovenia, who holds a clear lead despite placing only 18th Saturday, 1.74 back.
Like Vonn, Maze was fast at the top before a mistake midway lost her speed.