Hirscher leads nearest rival Aksel Lund Svindal by 31 points with only the slalom remaining, a race the Norwegian does not compete in.
Svindal failed to complete the first leg of the giant slalom, squandering his final chance to score points in the battle for the overall title.
Hirscher is the first male skier to win three straight "big globes" since American Phil Mahre in 1983.
Ted Ligety of the United States won the giant slalom, while Hirscher's victory made it an Austrian double, with compatriot Anna Fenninger securing the women's overall crown on Thursday.
World and Olympic champion Ligety claimed the final giant slalom of the season in nail-biting fashion, setting a combined time of 2:15.63, 0.03 ahead of France's Alexis Pinturault and 0.26 over Germany's Felix Neureuther.
He finished level on points with Hirscher in the discipline's standings, snatching the giant slalom trophy thanks to his five wins of the season.
"It's funny. Everybody is asking me whether I'm sad to lose the giant slalom globe but there's something so much bigger happening to me in the same time that I just feel mixed emotions," Hirscher said.
"The World Cup is always a challenge, but it's even stronger the third time because then you know what it means," he added.
Hirscher conceded his motivation might be faltering for Sunday's slalom, the last race of the season.
"It's such a weight off my back and I'm so tired, the battle cost me a lot. I believe Felix will have no problems to beat me tomorrow. In a way, my job's done," he said.
"Don't blame me if I don't ski at my best tomorrow. I'm going through extreme emotions right now and I can't really think about tomorrow," he added.
Ligety made history by winning his fifth giant slalom this campaign, one more than past greats of the discipline like Alberto Tomba and Michael Von Gruenigen.
"What happened today was just madness. I knew it was going to be tight but I never expected such a suspense. It's my fifth globe but none had the same taste. The first one was probably the one that mattered most but this one is just behind, it was so hard to win," the American said.
Ligety's compatriot Mikaela Shiffrin, a world and Olympic gold medallist, achieved a similar sweep, clinching her fifth slalom victory of the season to secure the slalom crown.
Two days after her 19th birthday, she left nearest rival Frida Hansdotter of Sweden more than a second behind.
"I had a hell of a season and I couldn't have dreamt of a better race today. I skied without pressure and I can really say I controlled it from start to finish," she said.
Austria's Marlies Schild was third and hinted at retirement at the end of a season which saw her beat the record number of World Cup slalom wins on 35.
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- giant slalom
- Aksel Lund Svindal