Rebensburg, who had never won a major race, clocked a two-run combined time of 2 minutes, 27.11 seconds down Franz's GS, and it held up when the first-run leaders were slowed by fog that got worse after her run.
Tina Maze of Slovenia was second, 0.04 second behind, matching her result in super-G, and first-run leader Elisabeth Goergl of Austria added another bronze, 0.14 back, duplicating her downhill finish.
The 20-year-old Rebensburg stood only sixth after the opening leg.
"Unbelievable! Unbelievable! Unbelievable!" Rebensburg said, adding that it helped having super-combined winner Maria Riesch and giant slalom world champion Kathrin Hoelzl on her team. "I had not that much pressure."
Hoelzl finished sixth and Riesch was 10th.
"She should experience this moment right now in the moment, because it all goes by like a film and tonight she will shake her head and wonder what happened," Riesch said.
Defending champion Julia Mancuso of Squaw Valley, Calif., who was 18th after the first run, had the third-fastest time of the second leg and finished eighth.
The first run was completed Wednesday, but dense fog forced organizers to postpone the second leg for a day.
Rebensburg's best previous result was second in the last World Cup GS before the Vancouver Games in Cortina d'Ampezzo, Italy. Before that, she had never finished on the podium, but she had been threatening to break out for several seasons. She finished eighth in giant slalom at the 2007 world championships in Are, Sweden, when she was just 17, and was ninth at last year's worlds in Val d'Isere, France.
The postponement, plus an early start time of 9:30 a.m. PST, took some of the drama out of the race. Many fewer fans were on hand compared to Wednesday, and even Rebensburg barely celebrated after finishing her run, assuming that one of the five remaining skiers would beat her.
Maze and Goergl both lost time at the beginning of their second runs, with the fog worst on top, then made up time on the bottom - but not enough to beat Rebensburg.
Only after Goergl came down did Rebensburg start pumping her fists.
Like the first leg Wednesday, organizers again rushed to get the race in, sending down racers at 60- and 75-second intervals - meaning there were often two skiers on the course at a time - and abandoning TV breaks before the first 30 racers came down and the medals were decided.
The short intervals created problems in the first leg. Mancuso's first trip down was interrupted Wednesday because teammate Lindsey Vonn crashed out immediately ahead of her - breaking her right pinkie. Mancuso had to be brought back up for another try but couldn't match the speed of her aborted run and was too far back to have a realistic chance in the delayed final leg.
While she won the giant slalom at the 2006 Turin Games, Mancuso entered these games with low expectations after a two-year dry spell. After taking silver in downhill and super-combined, she leaves the games content.
"It's been a long couple weeks, and the good thing about yesterday is I still have the pride of my two silver medals," Mancuso said. "There's lucky days, unlucky days. ... I'm psyched I was able to lay down a pretty good second run today, and it wasn't enough, but I really went out and did my best, so I'm proud of that."
Mancuso turned down an offer from U.S. coaches to race Friday's slalom, the final women's Alpine event of the games.
The race was briefly interrupted Thursday after 18 skiers came down due to an apparent timing equipment problem.
Maze has had a stellar games, also finishing fifth in super-combined.
"It's great. I didn't expect that I would take two medals," Maze said. "I was never racing so good at the big events, but this time it's different, and I skied confident."
Goergl's medal haul now matches that of her mother, Traudl Hecher, who also won two bronze medals at the Olympics - in downhill at the 1960 Squaw Valley Games and the 1964 Innsbruck Games.
Goergl also won a bronze in super-combined at last season's worlds, and she was already claiming family bragging rights.
"With my worlds championships medal it's five," said Goergl, whose older brother Stephan is also a World Cup skier.
Goergl said the overnight wait didn't affect her.
"I had the same focus as if we had started the race today," she said. "I wanted gold today."
Fabienne Suter of Switzerland finished fourth and Kathrin Zettel of Austria was fifth.
Giant slalom standout Tanja Poutiainen was 13th, Sarah Schleper of Vail, Colo., 14th, and Swedish standout Anja Paerson 22nd - even though her father and coach, Anders, set the second run.
The other American finisher was Megan McJames of Park City, Utah, who was 32nd in her Olympic debut.
Rebensburg's win gives Germany eight gold medals at the Olympic Games. The United States and Canada both have seven. The U.S. still holds a slim lead in the overall medal count with 28.
In other Olympic action Thursday:
U.S. BOBSLEDDER DETAINED
American bobsledder Bill Schuffenhauer was detained and released by Canadian police, although it's not yet clear why, the U.S. Olympic Committee said Thursday.
Schuffenhauer is a pushman for one of the three U.S. bobsled teams. A team coach expects him to compete in the four-man competition.
Canadian police declined comment, citing privacy laws.
Norway won the women's cross-country relay Thursday after Marit Bjoergen quickly decided the race on the final leg for her third gold medal of the Vancouver Olympics.
Bjoergen and Italy's Sabina Valbusa went out together at the final exchange, but the Norwegian immediately pulled away from her only remaining rival and skied alone the rest of the way.
The Norwegian team of Vibeke Skofterud, Therese Johaug, Kristin Stoermer Steira and Bjoergen finished the 4x5-kilometer race in 55 minutes, 19.5 seconds.
Germany was second after Claudia Nystad beat Finland's Aino-Kaisa Saarinen in a two-way race for the silver. Finland took the bronze and Italy was fourth after Valbusa faded.
In the ongoing dispute over whether the NHL will let its players participate in the 2014 Sochi Games, the head of Russia's pro hockey league says it would be a serious mistake for the NHL to stand in their way.
Alex Medvedev said he met three times in Vancouver with NHL commissioner Gary Bettman. Bettman is concerned about shutting down the NHL season during the Olympics. He also has doubts about an Olympics in which the hockey would be played at a time when many North American fans would be asleep.
VANCOUVER: IN MEMORIAM
IOC president Jacques Rogge says the death of a Georgian luger will forever be associated with the Vancouver Games, just as the slaying of Israeli athletes remains a legacy of the Munich Olympics.
Rogge says the International Olympic Committee accepts "moral responsibility" but not legal responsibility for the death of Nodar Kumaritashvili. He added that the incident should be remembered separately from the overall success of the games.
Rogge has urged Russian organizers to make sure the sliding track is safe for the 2014 Sochi Games.
It'll be Sweden vs. Canada in the women's finals.
The defending gold medalists from Sweden KO'd the reigning world champion Chinese in one semifinal, and the hosts took out Switzerland on a late shot.