Sullivan, a two-time Olympian who has been on the U.S. Ski Team for nine seasons, finished in 2 minutes and 1.1 seconds to edge World Cup downhill leader Didier Cuche of Switzerland by four-tenths of a second.
The U.S. Skiing Association described the French course as hard and fast, which favored Sullivan's style.
"I wasn't sure if my run would be good enough to win," said Sullivan, 27, Squaw Valley, Calif. "I had a couple of small scrubs (of time) on top, but I knew this was my kind of course and I knew I had a good run."
Slovenia's Andre Jerman placed third, while Sullivan's fellow Yank, Bode Miller, came in sixth.