McKennis, 23, tamed the slightly soft course in 1 minute, 14.62 seconds to edge Italian Daniella Merighetti by a scant 7/100ths of a second.
"It was a really tough course and I had to be really aggressive the whole way and never back off," said McKennis, whose previous best in the downhill was a seventh place. "If you do, it will take control of you."
Anna Fenninger of Austria placed third and Tina Maze of Slovenia was fourth in her best downhill finish of the season. Maze is the overall points leader in the ladies World Cup.
Americans Laurenne Ross and Lindsey Vonn took fifth and sixth place respectively. Vonn, the leader in the downhill points race, was timed in 1:14.96.