The 18-year-old overtook first-run leader Petra Vlhova to beat the Slovakian skier by 0.34 seconds in Kranjska Gora after producing a scintillating second run to take the top step.
Behind Robinson and Vlhova, Slovenia's Meta Hrovat and Switzerland's Wendy Holdener shared third place as they both earned a second career podium result in giant slalom.
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Vlhova shared victory with Federica Brignone in the previous giant slalom in Sestriere a month ago but the Italian skier, who sits top of the GS standings, could only manage eighth.
Brignone was able to close the gap to Shiffrin in the overall standings to 113 points though, while two-time former world champion Tessa Worley was ninth on her return to action.
The action had been scheduled to take place in Maribor but was moved to Kranjska Gora, which last hosted women's WC races in 2018, because of a lack of snow and mild temperatures.
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Petra Vlhova leads the way after first run