Shiffrin edges closer to all-time record after World Cup opener
Shiffrin was third after the first run, 0.55 seconds off the pace, but produced a superb second run to clock a combined time of 1 minute 51.25 seconds.
That was good enough to beat Sweden's Anna Swenn Larsson by 0.16sec and claim her 75th victory on the World Cup circuit.
At 27, the American is fast approaching the overall alpine ski record of Swede Ingemar Stenmark, who claimed 86 wins, and the women's record of 82 held by her compatriot Lindsey Vonn.
Shiffrin's great Slovak rival Petra Vlhova, Olympic champion in the discipline in Beijing earlier in the year, finished third, 0.20sec behind the winner.
Germany's Lena Duerr, who finished third in the overall slalom standings last season, set the fastest time in the first leg but a sluggish second run meant she could only finish fourth, ahead of Switzerland's Wendy Holdener.
Levi, a city in the far north of Finland, located above the Arctic Circle, marks the opening of the women's season after the cancellation in October of the giant in Solden because of the weather.
A lack of snow put paid to the next three events, two in Zermatt/Cervinia in Switzerland and then in Lech in Austria.
The women will face another slalom race in Levi on Sunday.