Both the gold and bronze medals went to American snowboarders, who brought some light to an otherwise gloomy day around the Olympic venues for the U.S. contingent.
The women's halfpipe was a unique competition in that the last three Olympic champions were all entered and all made the finals. None of them won.
Farrington, 24, has undergone three knee surgeries, five operations on a wrist and another on a thumb and still has come back for more in a sport known for its danger. Her biggest achievement prior to Wednesday came at last year's world championships, where she finished second.
She was second after the first of two performances in the finals Wednesday, but her second-round score of 91.75 put her atop the leaderboard.
None of the sport's leading lights could top it, although the second, third and fourth-place positions were all taken up by former gold medalists.
Australia's Torah Bright, the defending champion, was second with a 91.50. American Kelly Clark, who won the gold medal 12 years ago in Salt Lake City, finished third at 90.75. And Hannah Teter, the 2006 winner in Turin, settled for fourth at 90.50.
Teter was the only one among the top four whose final score was recorded in the first round, at the end of which she had the lead.
Farrington's win followed by 24 hours the disappointment of Shaun White failing to win a medal in the men's halfpipe. The American was the overwhelming favorite to win his third Olympic gold medal in a row.
U.S. speed skating star Shani Davis also faded earlier Wednesday in his bid to win a third consecutive gold in the 1,000-meter race.
All three of the medals won by United States athletes have come in snowboarding. The other two were won in the men's and women's slopestyle competition, which made its Olympic debut this year.