The 21-year from old from Vail, Colorado matched the accomplishment of German Christl Cranz who completed her treble in 1939, finishing a staggering 1.64 seconds ahead of home crowd favourite Wendy Holdener of Switzerland.
Stunned by the victory margin, Shiffrin jumped with joy on her skis after crossing the finish line and received a rousing ovation from the 25,000-strong crowd.
"I've had lots of challenges and lots of amazing days and today was a really good day," Shiffrin, the 2014 Olympic champion, told reporters.
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"I am very happy to go home with two medals."
It was a second podium finish for overall World Cup leader Shiffrin who also took the silver medal in Thursday's giant slalom.
Swede Frida Hansdotter climbed from fifth to third with a strong second run before Holdener, second after the first leg, lit up the fan zone in the finish area with a gritty run which edged her ahead by 0.11 seconds.
But any hopes of an upset the Swiss might have had were extinguished by a nerveless run from Shiffrin on a challenging course, which several pace-setters failed to complete on icy snow.
It was also a second medal of the championships for Holdener who won the combined event last week.
The 31-year old Hansdotter was happier with her result than the performance and heaped praise on Shiffrin.
"She is unbelievable," said the Swede after capturing her third world championship slalom medal, having taken the 2013 bronze in Beaver Creek and the 2015 silver in Schladming.
"She is so young and talented but it's unbelievable how good she is skiing.
"Today was a fight with icy and aggressive snow conditions and I am not really satisfied with my skiing but for sure I am with the bronze."
The championship ends with the men's slalom on Sunday when Austria's World Cup winner and overall leader Marcel Hirscher will be the hot favourite to complete the double after winning Friday's giant slalom.
(Writing by Zoran Milosavljevic; Editing by Ed Osmond)
