German rival Emma Aicher, who had started the day on the Kvitfjell slope 28 points behind the 28-year-old, finished fifth and 83 points off the lead.
American Breezy Johnson, the world and Olympic champion, finished second in the race -- 0.15 seconds off the pace -- to take third overall and Germany's Kira Weidle-Winkelmann was third in the race.
In the overall World Cup, Aicher reduced the lead of American Mikaela Shiffrin, who does not race downhill, to 95 points with three races remaining. Each win is worth 100 points.
Vonn led standings until March 7th
The downhill standings had been led by Lindsey Vonn until March 7, but the US ski great's bid for another globe at the age of 41 ended when she crashed and broke her leg in February's Milano-Cortina Olympics.
The win, on her father's birthday, was Pirovano's third in the World Cup and third in a row after two on one weekend on home snow in Val di Fassa.
Pirovano, who started with bib number 14 after Aicher had gone 11th, ended the downhill season with 536 points to the German on 453 and Johnson on 413.
"I felt honestly not so good," Pirovano said. "l was afraid to watch the time once I crossed the finish line.
"I was so surprised to see the green light."
It was a double success for Italy after Dominik Paris earlier won the men's downhill.
That race had no impact on the top of the standings, with Switzerland's Marco Odermatt crowned downhill and overall World Cup champion.
For Paris, 36, it was his 25th World Cup race win and 20th in downhill.
Switzerland's triple Olympic gold medallist Franjo von Allmen finished second in the race, 0.19 slower than Paris, with Austria's Vincent Kriechmayr third.
The men and women race the final super-Gs of the season on Sunday, with Shiffrin expected to be in action as she chases a sixth Overall Globe.
Odermatt has won the men's super-G Globe and Italy's Sofia Goggia is on course to secure the women's one.
New Zealand's Alice Robinson, Goggia's closest rival 63 points behind, did not start Saturday's race, which was the last downhill for Slovenia's retiring double world champion Ilka Stuhec.
