It was the 21-year old American’s second triumph in as many days after also winning Friday’s giant slalom whilst it was her 11th World Cup victory this season.
Shiffrin has now won the slalom title four times from the past five seasons.
In California Shiffrin was just over a second faster than Czech skier Sarka Strachova in second while Austrian Bernadette Schild rounded out the podium in third.
Switzerland’s Wendy Holdener was leading the event after the first run and was setting a blistering time in the second.
But with the line and victory almost in touching distance the 23-year old crashed out of the race leaving a stunned Shiffrin to claim the win.
Shiffrin headed into the second run knowing whatever her result she would win the slalom title.
Her closest challenger in the season-long slalom battle Veronika Velez Zuzulova of Slovakia, wiped out right before Shiffrin took her turn.
After this weekend, there are four races remaining at the World Cup Finals in Aspen, Colorado where Shiffrin, who turns 22 on Monday will look to also secure the overall world cup crown. (Euronews)