After an hour's delay due to severe weather conditions including poor visibility, the competition finally got underway in heavy snow, which broadcaster and Olympian Kelly VanderBeek described as the worst she'd seen at Lake Louise in 20 years.
Ledecka, wearing bib number 26 as she did in South Korea, stunned leader Corinne Suter of Switzerland who was hoping to claim her first career World Cup victory.
Best of the rest
Suter, the 2019 world silver medallist, was 15th to race and led at all but one of the intermediate points until Ledecka put in her incredible run and finished with a 0.35-second margin.
Austria's Stephanie Vernier was third to record her fifth career World Cup podium, 0.10 seconds behind Suter. Vernier overtook early leader Viktoria Rebensburg, the first woman down the slopes.
Olympic downhill champion Sofia Goggia of Italy finished sixth.
Mikaela Shiffrin, the double Olympic champion, was sixth to leave the start gates but was unable to overtake Rebensburg, who held the lead through the first six racers. The American finished 10th.
Meanwhile, in the men's Super-G held at Beaver Creek in Colorado, the United States, Switzerland's Marco Odermatt clinched his maiden World Cup win, becoming the youngest winner of a speed World Cup race since 1994.