On crossing the finish line, Shiffrin gazed up for several seconds at the giant screen and then gulped in air on finally seeing the size of her victory.
"I didn't want to celebrate until I knew for certain I had won," she said. "But when I saw 1.6, I thought, 'Is the time wrong?' That was a really special moment."
Swiss fans, who had loudly supported Holdener two minutes earlier, duly acclaimed the unbeaten champion.
"I heard the crowd cheering so loud," Shiffrin recalled of her wait to start. "I was thinking, 'Oh that's cool, (Wendy) must be happy, but today is my day, not yours.'"
Minutes before her decisive run, Shiffrin was filmed keeping cool by sitting beneath an umbrella to find shade from the sunshine.
Her gold medal streak in slalom now includes each world championships she entered, and her only Winter Olympics.
Shiffrin's victory also gave the United States its first world title at St. Moritz in the 10th of 11 medal events.
On a clear day and in perfect racing conditions, Shiffrin was 0.85 faster than any rival second time down.
Holdener added slalom silver to her gold medal in the combined event. Both her medals were won after Swiss star Lara Gut crashed out of the championships with a serious knee injury.
"It's crazy," the 23-year-old Holdener said. "It wasn't always easy, the pressure is here. I'm proud of myself."
With Shiffrin also getting a silver medal in giant slalom, the U.S. team's tally is three. Lindsey Vonn took bronze in downhill.
"I'm really excited with the silver in GS and today's race is as good as it could be," said Shiffrin, who was congratulated course-side by IOC President Thomas Bach.
Bach watched Shiffrin's second run from the same VIP seats where Roger Federer saw Vonn race last Sunday.
"I think I am her best mascot," Bach told The Associated Press. "Whenever I am at (ski) world championships, she is winning. So we were looking forward together for next year."