Vonn turned in a 1:22.16 on the icy course in Garmisch, edging Sweden's Anja Pearson, who turned in a 1:22.55.
Another Swede, Jessica Lindell-Vikarby, came in third.
Garmisch is the final World Cup tune-up before next week's world championships. The men's downhill was scheduled to be run later Sunday.
Vonn said that while the course was set up in a way that made bursts of dominating speed few and far between, she was able to pick up some tips from her skier husband, Thomas, who was familiar with its quirks and tough spots.
The victory was Vonn's 18th World Cup win and tied her with Tamara McKinney for the most wins in a career by an American woman.
Celebrity Couples of 2014 [PHOTOS]