Shiffrin finished 0.76 seconds ahead of Slovakia's Petra Vlhova (27) to collect her 81st World Cup triumph in 1:36.42.
The three-times Olympic medallist will have the chance to equal Vonn's record on Thursday under the lights once again.
"I'm incredibly happy, I had so much fun skiing today and it was my best skiing both runs," Shiffrin told FIS TV.
"Nothing less than the best is going to work and I was taking all the risk I needed. It's an amazing feeling when it's good enough."
Shiffrin lies third on the all-time list for the most World Cup wins, behind Sweden's Ingemar Stenmark (86) and Vonn.
She tops the overall World Cup standings with 975 points, a massive 389 ahead of Vlhova. In the slalom standings, Shiffrin (525) leads Wendy Holdener (29) by 105 points.
Shiffrin is seeking a fifth overall crown which would put her second on the all-time list behind Austria's Annemarie Moser-Proll who has six titles to her name.
