The overall World Cup holder, who had been on the podium four times so far, added a 13th win to his career record in a combined time of one minute 54.10 seconds.
"The first win of the season is always a relief and a turning point, for me and for the fans, but I had had four podiums already this season and I knew it was only a matter of time," he told reporters.
Third in Saturday's slalom on the same Bellevarde piste, Hirscher was too strong and consistent for the rest of the field, even though local favourite Alexis Pinturault of France challenged him until the end.
The two had opened up a big gap on the other skiers in the morning run and Pinturault, winner of the previous day's slalom, was still comfortably leading in the second leg before a big mistake four gates before the line ruined his chances of a home double.
"Yesterday was a celebration, today a big disappointment. I hope to make it back on the podium in Alta Badia next week," said the Frenchman, arguably the most versatile skier produced by his country since the days of Val d'Isere's most famous citizen, Jean-Claude Killy.
Hirscher himself is at home in the French resort, having won his maiden World Cup victory on the course in 2009.
"I love it here. I know it's a slope that suits me and that I can tackle with a lot of confidence," he said.
German Stefan Luitz, who had only two top-30 placings behind him, was second, 1.16 seconds adrift, while world champion Ted Ligety struggled to find his pace on the steep piste and had to be content with third, 1.41 behind the winner.
Luitz, a junior world silver medallist, started with bib number 35 and was 25th after the first leg but the 20-year-old German clocked the fastest second leg by far to clinch his first significant result in the top flight.
The Val d'Isere weekend confirmed the rise of a new generation, with Hirscher, Pinturault and Luitz all aged under 23.
The 28-year-old Ligety, who dislikes the Bellevarde course, was not too disappointed.
"I don't like this course which is against my philosophy of giant slalom. It was ugly skiing but it was good enough to be on the podium," said the American, adding that he had big ambitions for next weekend's super-G in Val Gardena, Italy.
In the overall World Cup standings, Hirscher moved back up to third, 60 points behind Norway's Aksel Lund Svindal and 22 behind Ligety, who won the first two giant slaloms of the season.
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- Alexis Pinturault