Shiffrin has now won four out of four women's slaloms this season, and five in a row, including the last of the 2024-25 campaign.
The American, very much the golden favourite for next February's Milano Cortina Olympics after failing to medal in Beijing in 2022, was 0.83 of a second quicker than closest rival Camille Rast of Switzerland after the first run of the night race.
She was only equal fifth fastest on the second floodlit run but still crossed the line with a combined time of one minute 42.50 seconds, 1.55 quicker than Rast.
Germany's Emma Aicher, winner of a downhill in St Moritz on Saturday, was third.
"I am just pushing, I'm not asking questions," Shiffrin, who has won all four slaloms by more than 1.2 seconds, told FIS TV when asked what her secret was.
"Sometimes you've just got to take it and roll with it. And these women are pushing, they are pushing like hell, so I have to push too.
"It's a wonder that I made it to the finish in time."
Shiffrin said she still had work to do on her giant slalom form, and she also needed to train more for Super-G after getting a thrill out of skiing her first in two years over the weekend.
Teammate Lindsey Vonn, who at 41 became the oldest ever World Cup winner when she triumphed in a St Moritz downhill on Friday, is the closest female skier to her tally with 83 victories.
Retired Swedish great Ingemar Stenmark won 86 on the men's tour.
While Shiffrin continued to set the pace, rivals fell short -- a series of top names straddling gates or skiing out.
Germany's Lena Duerr, second in Copper Mountain, was third after the first run but straddled on the second after Aicher had gone top.
Italian-born Lara Colturi, who skis for Albania and has had three successive podiums, failed to finish the first run.
So too did Croatia's defending World Cup slalom champion Zrinka Ljutic, a non-finisher for the third race in a row, and Austria's 2022 Olympic silver medallist Katharina Liensberger.