Schild, the slalom world champion, finished in one minute 51.53 seconds to secure the 30th slalom win of her career. She had also won the season opener in Aspen, Colorado in November and the race in Courchevel, France, two days ago.
Defending overall World Cup champion Maria Hoefl-Riesch of Germany was 0.46 seconds behind in second, and world giant slalom champion Tina Maze of Slovenia took third, 0.82 behind, for her first podium finish of the season.
"Thirty, what an amazing number of wins that is," Schild said. "I still don't take anything for granted. Every race is different, every course is different. When you win, you're sure that you're fast, but it's no guarantee for the next race. Every time you have to focus again."
Marie-Michele Gagnon of Lac-Etchmenin, Que., was ninth in 1:53.69 and Erin Mielzynski of Guelph, Ont., was 14th in 1:54.19.
"I’m really happy with the result but this year I want more," Gagnon said. "I keep wanting more. I’m slowly getting my start numbers down. It’s getting really exciting but I know I can do more. I think I still have to learn to really go for it."
Schild is rapidly closing on in the all-time record of 34 slalom wins by Swiss great Vreni Schneider.
"Before the season, I was not thinking about it," Schild said. "But with the way things have been going so far, it has come into my mind now."
Lindsey Vonn finished 2.01 seconds back in eighth and remains top of the overall standings.
The American has 554 points, 230 clear of Schild, who is not a contender for the overall title as the Austrian only competes in technical races. Germany's Viktoria Rebensburg is third with 286 points and Hoefl-Riesch is fourth with 271.
Schild won in Flachau two years ago but failed to live up to expectations last season.
"I started with mixed feelings because of last year's race here when I messed up the first run and skied out in the second," she said after her first run. "Now I am in the lead, that's great. I lost some time on the lower section where I was too close (to the gates) and lost some speed."
Schild avoided risks in her second run and barely lost any time from her first-run lead of 0.53 seconds over Hoefl-Riesch.
The German bruised her left knee in the Courchevel slalom on Sunday and decided only hours before the race in Flachau to start, ignoring a doctor's advice to rest for a week.
"Maybe it would have made sense to skip this race and get healthy for next week, but I love the races here," said Hoefl-Riesch, who earned the last of her seven career slaloms victories in Flachau last year.
"It's a mental problem when you know something is not right," she said. "You saw it in the first section (of the first run) where I raced a bit conservative, I lost a lot of time there. After that, I managed to get it out of my mind."
Hoefl-Riesch said the injury did not hamper her while racing but her knee hurt between runs.
"I am really proud to finish second under the circumstances," she said.
Hoefl-Riesch said she wasn't racing just to avoid losing more ground in the overall standings.
"I am not looking at the points right now," Hoefl-Riesch said. "I am that far behind for the overall that I am only focusing on one race after another."
Vonn was pleased with her first top-10 finish in slalom since finishing eighth in Aspen a year ago. The American missed this season's first slalom race in Aspen with back problems and did not finish her first run in Courchevel.
Vonn, who has not been on a slalom podium for more than two years, had a decent first run without obvious mistakes but lost her rhythm once at the halfway stage of the second.
"I skied really well on the top but was a bit too conservative in the middle," said Vonn after her first run. "I am happy, it is a top 10 and I didn't make any mistakes really. For me it's important to finish and get my confidence back."
Vonn said her substantial lead in the overall standings allowed her to take risks in the second run.
"I have nothing to lose, I am in a good position for the overall right now," she said, adding that she wanted to "show that I can still ski slalom."
A men's World Cup slalom on the Griessenkar course is scheduled for Wednesday. The Flachau races replace the event that was cancelled in Levi, Finland, in November because of a lack of snow.