Despite finishing the first leg of the floodlight race in second place, Shiffrin charged down the course on her second run to win the race by an impressive margin of 0.94 seconds.
It was the 22-year-old's eighth victory in the last nine races in all disciplines, her form peaking at the perfect time with the Winter Olympics in Pyeongchang one month away, and the 41st World Cup win of her career.
Shiffrin is widely expected to retain the Olympic slalom title she won at the 2014 Games in Sochi, Russia, as well as having a shot at gold medals in the giant slalom and Super G.
'I’m trying to decide' - Shiffrin to make Winter Olympics decision
Austria's Bernadette Schild was fastest on the first leg by 0.37 seconds but, skiing last on the second, had already dropped behind Shiffrin's time on the first third of the course.
Mikaela Shiffrin à Flachau.
Image credit: Getty Images
Shiffrin said she had been nervous after finishing joint third at the same venue last year and was glad to have won a race after trailing on the first leg.
"Maybe I needed to do that tonight," she told Austrian broadcaster ORF. "I was a bit anxious about this race and I made some mistakes on the first run. On the second, I finally got what I wanted from this hill."
Shiffrin, who won her maiden overall World Cup title last season, tops the overall standings with 1,381 points, a lead of 821 over second-placed Wendy Holdener of Switzerland.
World Cup wrap: New era finally here after season where anything was possible
Robinson's sizzling run denies Shiffrin in final giant slalom of season