Vonn finished in 1 minute, 52.61 seconds to beat American teammate Stacey Cook by 1.73 seconds. Germany's Maria Hoefl-Riesch and Liechtenstein's Tina Weirather tied for third in 1:54.35 in the race interrupted a few times because of fog and poor visibility.
Vonn, a part-time Park City resident, raced for the second time since returning from a stomach bug that landed her in the hospital. Last week in Aspen, Colo., she was 21st in the giant slalom, then skipped the slalom.
"It seemed like everything was going wrong," Vonn said. "It's hard sometimes to believe that you're going to turn things around. I just keep fighting and every day, I keep trying my best. I was just so happy I was able to come out today and ski the way I wanted to. It was just a big sigh of relief."
The 28-year-old, a four-time overall World Cup champion, will race another downhill Saturday and in a super-G Sunday. She swept the three races last year.
"The only thing I'm concerned about at this point is my energy level and trying to make sure I have enough energy for the next couple of days," she said. "It was a long time standing out in the cold today and hopefully I can just get some good recovery in today and maintain what I have and be able to ski the way I want to."
Vonn won after skipping a training run Thursday because of more stomach pain.
"I didn't do the training run, which I've never done before in my life, so I was nervous today. I know this hill better than anywhere else in the world, and I gave it my best shot and was really happy with my run."
Cook, from Mammoth Mountain, Calif., collapsed into her mother's arms and cried when she realized she would stand on a World Cup podium for the first time in her career.
"Someone told me this was my 150th World Cup start. One hundred and fifty tries to get on the podium," Cook said. "Nine years of it. It's a long time coming and my parents, they come to one race every three years. For them to choose this one was really special.
"I've battled so much in my career and most of the battles have been within myself, just figuring out how to compete with these girls that seemed to be so much more gifted than me, and trying to overcome that."
Christof Innerhofer of Italy glided through a demanding course at Beaver Creek, Colo., to win his first World Cup downhill in nearly four years. Innerhofer completed the Birds of Prey course in 1 minute, 41.69 seconds, holding off Norway's Aksel Lund Svindal by 0.23 seconds. Norwegian Kjetil Jansrud was third.
Defending champion Bode Miller watched the race from a seat near the finish area. He is recovering from offseason surgery on his left knee.
Innerhofer, 27, said he could barely train in the offseason because of his chronically aching back.
"I shouldn't train maybe I should just take a holiday," he said, laughing. "I'm crazy happy right now, because I really can't believe it."
His last downhill win was Dec. 28, 2008, in Bormio, Italy.
Svindal was was coming off two wins in Lake Louise last weekend.