She became the first female to break the 56-second barrier, lowering the record Sweden's Sarah Sjostrom (56.06) set in 2009.
"There've been so many moments in my career when I didn't know if I could keep going," said Vollmer, 24. "Standing up there [on the medal podium], it paid off, we did it, we had made it, and I did something no one's ever done before."
Vollmer's first 50 meters Sunday didn't separate her from the field. But she made the turn with an underwater surge that is her specialty, surfaced ahead, and shot toward home, hunting the record. Soon after, she bent her head to accept the gold medal around her neck and sang along to the national anthem, her long, blond hair still wet on her shoulders.
China's Lu Ying finished second in 56.87, and Australia's Alicia Coutts placed third in 56.94.
The world record was just the second by a female since high-tech suits were banned on Jan. 1, 2010. Two other Olympic records fell Sunday in the women's competition.
Vollmer is a different person now than the broken college student who left the 2008 U.S. Olympic trials in Omaha crushed. She had failed to make the U.S. team in any event, finishing just fifth in the 100 fly, her event, her confidence shaken by nerves and health problems.
"I look back at 2008 and I wasn't excited to compete," Vollmer said. "I was more worried about what happened if I failed. . . . I crumbled under all that."
Teri McKeever, Vollmer's coach since she attended California and the U.S. women's coach here, found a way to start putting Vollmer back together. She shipped her from her Bay Area home off to Fiji.
Able to block out the ongoing Beijing Games she hadn't made, Vollmer taught children how to swim and trained in the ocean, competing in her first open-water race. It was a start.
"It made me realize I loved being in the water and I loved swimming," Vollmer said. "I had a blast."
French win in 400 free
Camille Muffat of France edged Allison Schmitt of the United States by less than half a stroke to win the Olympic 400-meter freestyle Sunday.
Muffat clocked an Olympic-record 4 minutes, 1.45 seconds, while Schmitt was second in 4:01.77.
Defending champion and local favorite Rebecca Adlington made a late charge to take bronze in 4:03.01 for Britain's first swimming medal in the London Games.
Two-time world champion Federica Pellegrini of Italy finished fifth.
This article contains information from the Associated Press.