American puts in stunning slalom run as Italy's Frederica Brignone, who led after the Super-G, slides off after just three slalom gates
The American secured her sixth world title – and first in combined – with an attacking approach to the icy slalom course on the Olympia delle Tofane, which suited her style of skiing more than some of her rivals'.
It made her the most-decorated American skier in World Championships history, with her ninth medal (six golds, one silver, two bronzes) moving her clear of Lindsey Vonn's eight total medals and Ted Ligety's five golds.
A third-place finish after the first Super-G run put the 25-year-old in a good position, and she made no mistake on the slalom. Shiffrin's 45.05-second slalom run gave her a total time of two minutes, 7.22 seconds in her combined debut at the Worlds.
Petra Vlhova, Shiffrin's great Slovakian slalom rival, started with a deficit of 0.34 seconds to Shiffrin and tried hard to make it up, but settled for silver 0.86 seconds behind.
Swiss Michelle Gisin, the Olympic champion in the combined who was fifth after the Super-G, improved to third a further three one-hundredths of a second back from Vlhova.
Shiffrin's route to gold became clearer when Federica Brignone – the winner of the last four Combined World Cup races and leader of the race after the Super-G – slid off in the twisty, tricky opening sequence of gates. That same sequence also claimed the defending champion Wendy Holdener of Switzerland.
Brignone's unfortunate Did Not Finish result continued a poor home World Championships for the Italians, who also saw Elena Curtoni finish fourth just outside the medals.
"I'm happy that we decided to put this in the plan this World Champs," Shiffrin said to FIS TV of racing in the combined for the first time at Worlds.
"It was a really nice day, beautiful weather and really great tracks in both events and for sure it was a really tough slalom, so I'm happy I got down with a good run.
"When you look at the top finishers, it's the girls who are strong in both events. It's definitely a tough slalom.
Commenting on the icy conditions, which led to barely half the field – just 16 of the 30 starters – finishing the slalom, Shiffrin said: "The conditions, they were well-prepared.
"It's absolutely necessary because the men are coming down skiing on the same track so it's fair for the whole field.
"But it's sometimes intimidating to go on that surface and think it's like going into battle. You have to get really psyched up in order to do the best skiing."