Ligety is the first American man to win two Alpine skiing golds following his super combined title in Turin in 2006.
"This is the event I wanted most," Ligety said. "It's the event that I've been putting so much pressure on myself to win, so to pull through is an awesome feeling. I'm super happy."
The 29-year-old world champion put nearly a second on his rivals in a smooth-as-ice first run through the gates on a crispy Rosa Khutor piste - and how he needed that buffer later.
French duo Steve Missillier and Alexis Pinturault roared into medal contention with their second runs before Ligety, the last of the top 30 to go, blasted out of the starting gate.
When the American slid sidewards almost to a standstill at one devilish part of the rutted and bumpy course the gold seemed in danger of slipping through his fingers.
Another mistake would have been calamitous but Ligety regained his rhythm to seal gold, 0.48 seconds ahead of silver medallist Missillier and 0.64 in front of Pinturault.
Ligety's victory gave the U.S. its first Alpine skiing gold of these Games while France celebrated its first medals of any colour on the slopes of Rosa Khutor.
"We're there, it's beautiful. We knew that after the first leg we had to go full gear, that's what I did and it worked," Missillier said.
"Vice Olympic champion - it's huge, it has not sunk in yet. Sharing it with Alexis is fantastic."
Ligety came to the Games as the United States' big medal hope in the men's events after his three golds at last year's world championships. But after diappointing in the super combined and super-G the pressure was on in his favoured event.
He had won nine of his last 14 giant slalom races on the World Cup but the Olympic title has been his burning ambition since missing out in Vancouver four years ago.
"This what I've wanted since I was a little kid," he said. "But being favourite in Alpine skiing, it's never easy because it's an event that is so far from being guaranteed.
"It's not super simple to win even if you are skiing the best in the world."
Ligety, who was only 14th quickest in the second leg, lost time over the Bear's Brow section of the course - a jump which he said caught him out regularly in training.
"I knew how critical that was and, knowing that, I knew it wasn't worth taking any risks, you might as well come to almost a complete stop and ski the next three gates well.
"Crawling over that roll cost me some tenths but if I had caught some air I could have lost a second on the flat."
Austria's slalom specialist Marcel Hirscher finished fourth, while Ligety's team mate Bode Miller was well down in 20th. "What good is fourth to me, I might as well go home," he said.
The only skier to get anywhere near Ligety's time in the first run was Germany's Stefan Luitz, who crossed the line on one ski a half-second down but was later disqualified for straddling the final gate of his run.
Luitz's team mate Felix Neureuther, still suffering pain from the neck injury he sustained in a car crash last Friday, was well-placed in a pack of skiers looking for a podium place but could manage only eighth.
- Sports & Recreation
- Rosa Khutor
- Alexis Pinturault
- Steve Missillier
- Alpine skiing