Hoefl-Riesch was fourth after the downhill portion before posting the second-fastest time in the slalom to finish in 2 minutes, 39.92 seconds. She beat Tina Maze of Slovenia by 0.46.
Former overall champion Nicole Hosp was fastest in the slalom and came one second behind in third to earn Austria a first medal at its home worlds.
"I can't believe it," Hoefl-Riesch said. "The last couple of weeks have not been easy. I had only two podiums this entire season so I wasn't coming here as a favourite. My goal was to win any medal, the colour didn't matter to me. Incredible, it is gold."
Hoefl-Riesch appeared to enjoy a clean slalom run, but she said it wasn't.
"I am surprised as my slalom didn't feel great," she said after her sixth career medal from major championships. "I was a bit too conservative. I didn't finish my last two slaloms on the World Cup so that made me cautious."
Maze had shared the lead after the first run with Austria's Anna Fenninger, who straddled a gate in the slalom. Lara Gut of Switzerland, who had the same downhill time as Hoefl-Riesch, also failed to finish.
"A classic mistake," said defending champion Fenninger, who was 1-2 with Maze at the worlds two years ago. "I don't train a lot of slalom so timing was always going to be difficult."
After a freezing night with clear skies and temperatures sinking to minus-15 (5 F), the course was harder and icier compared to the downhill training runs.
Racers had to deal with a rougher and bumpier surface, making the turning sections of the course even tougher to master.
Maze, who holds an almost insurmountable 888-point lead in the overall World Cup standings over Hoefl-Riesch, the 2011 winner, lost speed after an early mistake in her slalom, which was set by her coach Livio Magoni.
The Slovenian still earned her second medal of the worlds after taking gold in Tuesday's super-G.
"I am not satisfied with my slalom run," Maze said. "I held back and showed too much respect. My downhill was pretty good though. Second place and a silver medal is a super result."
Maze won the only World Cup super-combined this season in December and started as a clear favourite for the world title.
"I felt a lot of pressure at the start. I tried to stay calm but that wasn't easy," Maze said. "People keep asking me about the possibility of winning five medals at the world championships. That door is still open and I won't close it."
Hosp earned her fifth medal in total at world championships, but her first since 2007, the same year she won the overall title.
After years of struggling with injuries, the Austrian said she finally felt like her best form was back.
"I am glad I finally had a good downhill again, I finally skied like I used to do," Hosp said. "My slalom was excellent, that makes me really happy. I am so happy it worked out great. I had some tough times in recent years. And now I have a medal."
Hosp said winning Austria's first silverware at the worlds will take pressure off the team, which failed to medal in either the women's or the men's super-G.
"It was really hard for us with no medals after two races," Hosp said. "The whole nation is watching these races."
Lindsey Vonn won the World Cup super-combined title for the past three seasons but the American was out for the season after injuring her right knee in a crash at Tuesday's super-G.
Teammate Julia Mancuso, runner-up to Hoefl-Riesch in the 2010 Olympic race, finished eighth, 3.33 seconds off the winning time.
"That was a little disappointing. I was ready to have a good run," Mancuso said. "I still have that mentality that I want to try and win, so maybe I'm pushing it too hard."
Mancuso was 19th after the downhill but had the seventh fastest slalom.
Sofia Goggia of Italy placed seventh, 2.77 off the lead, to score another surprising result, three days after coming fourth in the super-G.
The 20-year-old Goggia has competed in only four World Cup races — all giant slaloms in which she failed to qualify for a second run. The Italian, however, has won four races, including a super-combined, on the lower-tier Europa Cup circuit.
The men's downhill is next on Saturday.