Shiffrin powers ahead of Holdener to claim second slalom win in Levi
The reigning overall World Cup champion finished 0.28sec ahead of Swiss Wendy Holdener with Olympic champion Petra Vlhova third at 0.68sec.
"I didn't expect such a result today after the first run," Shiffrin said after clocking the best time in the first run with a slim margin on her rivals.
Shiffrin won the overall World Cup title for the fourth time in the spring, but was frustrated by a failed Olympic Games in February in Beijing with no medals in three events.
But the 27-year-old from Colorado has managed the perfect start to the season with two victories in 24 hours.
The women's giant opener in Solden, Austria was cancelled at the end of October due to excessive rainfall, before three successive competition weekends - two in Zermatt and Cervinia and one in Lech - were scrapped due to lack of snow.
Behind Shiffrin, Holdener climbed onto her 30th World Cup podium in slalom but still has no victory, an unfortunate record partly due to the American's success.
It was Shiffrin's sixth victory on the tough Levi Black piste, to move ahead of Slovak slalom rival Vlhova with the pair having won every women’s World Cup race in the Finnish resort dating back to 2015.
The result also moves the American closer to becoming the most decorated World Cup skier in history with her 76th victory putting her six short of compatriot Lindsay Vonn, and ten off Swedish men's great Ingemar Stenmark's all-time mark.