"It's an unbelievable happy day," said Hirscher, content to concede the discipline to the American dubbed "Mr. GS" by race announcers.
Ligety earned his victory in a crowd-pleasing race by just 0.03 seconds ahead of runner-up Alexis Pinturault of France, in a two-run combined time of 2 minutes, 15.63 seconds.
"I'm super-thrilled. That was by the skin of my teeth today," Ligety said.
• American Mikaela Shiffrin completed her exceptional slalom season with a fifth World Cup win on Saturday in Lenzerheide, Switzerland.
The Olympic champion was 1.44 seconds faster than any rival in one of the longest women's slaloms in World Cup history. Shiffrin already clinched her second straight World Cup title in the discipline last weekend with a win in Sweden.
"I am very proud. One of my goals this year was to hold on to the slalom globe," she said.
She had a two-run combined time of 2 minutes, 4.54 seconds. Frida Hansdotter of Sweden was second and Marlies Schild of Austria was third.
"I did a pretty good job the first run I guess so I felt more comfortable the second run, and just trying to hold on to it," Shiffrin said.
Shiffrin led the standings from start to finish after winning the season-opening race in November at Levi, Finland. Her ninth career World Cup victory on Saturday came two days after her 19th birthday.