Americans Elana Meyers and Aja Evans took first place with a two-heat time of 1 minute, 37.67 seconds to edge Olympic gold-medal favourite Humphries and give the U.S. its first victory on the women's World Cup circuit in nearly three years.
Humphries led at the midway point, then lost some time in her second run with a mistake at the top of the track, and Meyers' took over the top spot.
"It's their home track so I knew competition was going to be tough," Humphries said.
Jamie Greubel and Katie Eberling of the U.S. were third, and Jazmine Fenlator and Emily Azevedo narrowly missed the American medal party, finishing fourth, 0.29 seconds behind Greubel and Eberling.
The last U.S. women's team to win a World Cup race was Shauna Rohbock and Valerie Fleming, who prevailed in Igls, Austria, in January 2011.
"I had a rough training week but I can't say enough about my teammates," Meyers said. "I got a text from every single one of them, telling me to keep my head up and telling me I could do this, and I'm so happy we came out here and took away the gold."
It's the second straight race where Humphries, Meyers and Greubel have driven to the top three spots. No sled from the powerful German women's team has a World Cup medal yet this season.
"It was a great race for Team USA ... definitely exceeded our expectations," Greubel said.
In the men's event, American Steven Holcomb stayed perfect so far this season.
Holcomb and Chris Fogt edged American teammates Nick Cunningham and Dallas Robinson for the gold, winning in 1:35.51. Cunningham and Robinson were 0.25 seconds back, while Germany's Francesco Friedrich and Jannis Baecker — the reigning world champions — were 0.29 seconds off the pace in third.
"When you have two great starts, it's easy to stay in the lead," said Holcomb, who calls Park City home and learned bobsledding on that very track.
Cory Butner and Chuck Berkeley were fourth for the U.S., 0.13 seconds off the bronze-medal time and adding to a huge day for the U.S. Bobsled and Skeleton Federation, which got two skeleton medals earlier in the day from Noelle Pikus-Pace (gold) and Matt Antoine (bronze).
"Home-track advantage ... and a great confidence booster," Cunningham said.
Bobsledding continues in Park City on Saturday with the four-man and another women's competition, before the circuit shifts to Lake Placid, N.Y. next week.