Hoefl-Riesch was in fourth place after the downhill Friday but completed the slalom segment in a career-best 50.05 seconds for a combined time of 2 minutes, 39.92, which edged World Cup overall points leader Tina Maze by 0.46 seconds.
Hoefl-Riesch had been en route to a flawless downhill when she made a slight mistake in the second half that slowed her down.
"I would have never thought to beat Tina in this race," said Hoefl-Riesch, who is second in the overall standings. "I didn't come here as a favorite but have reached my goal to win a medal."
"I knew it was going to be a really big fight for the gold medal today," Mancuso said. "Both courses were in perfect condition but I had a mistake in the downhill so I wasn't expecting much."