The 17-year-old Coloradan parlayed a solid first run into a combined time of 1 minute, 51.45 seconds and topped Sweden's Frida Hansdotter by 0.85 seconds.
The victory moved Shiffrin, last season's rookie of the year, into a fifth-place tie with fellow American Lindsey Vonn in the overall women's World Cup standings. Vonn and teammate Julia Mancuso did not participate on the night race, leaving it to Shiffrin to lead the U.S. effort.
"My secret is to not let anything stop me," Shiffrin said. "Sometimes the crowd gets into your head and all of a sudden you're nervous but instead I just let them take me to the finish."
The door opened for Shiffrin in the second run when first-run leader Maria Hoefl-Riesch of Germany failed to finish.
Veteran Tanja Poutiainen of Finland took third and overall points leader Tina Maze of Slovenia came in fifth with a 1:52.71.