Mills, 45, who lost her left leg in a road accident in 1993, was all smiles at the Games adaptive slalom medal ceremony last night in Queenstown's Games Village, saying it was an amazing result for her.
The ex-wife of Beatle Sir Paul McCartney said her speciality was downhill speed racing, not slalom, which involves skiing through a series of flags.
This was only her third adaptive slalom event.
Athletes aiming to make the British team for the Paralympic Games in Sochi, Russia, next year must get good results across a range of skiing disciplines - Mills is one of eight pushing for five British team spots.
A stoked Mills said: "It was massively important for me, so I'm over the moon. I never thought I would get a medal."
Ranked 28th in the world, Mills who skis with a prosthetic leg clocked a combined time of 2mins 9secs. Melanie Scwartz of the United States came in 10 seconds faster to win.
The pair survived a rough day on the Coronet Peak course, which saw just 26 of the 42 adaptive athletes make it through their first run.
"It was like a sheet of ice," Mills said. "You just have to chuck yourself down and hope for the best. The start pitch is so steep, you just freefall to your death basically to make the first gate if you don't push off fast enough, you haven't got a chance.
"It's quite scary when you're freefalling down. If you're not right on the edge of your skis, you're out."
Mills had plenty of praise for the volunteers and organisers for pulling the event off in tough conditions, saying they were all incredibly determined to ensure athletes raced.
"You could go to some countries and it would have been called off, but everybody here is so determined. We could have come all the way and the race could have been cancelled. The crew up the hill did a brilliant job."
Queenstown was amazing, Mills said.
She last visited the resort in summer more than 15 years ago when on a bike tour around the country.
"It's grown a lot just all the shops. It's beautiful.
"My coach is from Scotland and he said it's like Scotland on steroids."
Mills had little time last night to savour her result, with a second Games World Cup adaptive slalom race at Coronet Peak today.
"We want to but I can't celebrate as we have another race tomorrow.
Mills' coach John Clark says she skied close to her potential in the second run.
"There aren't words to describe what these athletes are all about. I'm relieved she's got this slalom result because she's rock solid in the speed events."
Mills said she could not wait to race in her preferred downhill speed events the Super G and Super Combined World Cup races at Canterbury's Mt Hutt field at the end of the month.
- Ryan Keen of the Mountain Scene
