Sinclair (18), of Wanaka, went into the halfpipe still dealing with the after-effects of an injured ankle, which had forced her out of competing in the snowboard-cross racing during the weekend.
However, she quickly put the recent injury woes behind her as she completed a high-scoring first run during the semifinals to claim an early confidence-boosting lead in the competition.
Sinclair progressed to the last round of six riders, but was soon left playing catch-up when the eventual winner, Cilka Sadar (18), of Slovenia, carded a 37.80 in her first run of the finals.
Sinclair landed a "crippler 7" (an inverted 720-degree spin through the air) in her second run, which boosted her score to 31.70 - enough to claim second, 0.3 points ahead of Haruna Matsumoto (17), of Japan.
Sinclair said her troublesome ankle had created doubt during her training and lead-in to the halfpipe.
"I wasn't even expecting to be able to compete with my injured ankle.
I've been struggling with landing the crippler all week and I'm stoked I pulled it off in my second run," she said.
Fourteen-year-old Japanese snowboarding sensation Taku Hiraoka's aerial form and amplitude above the halfpipe lip proved too much for a high-class field on his way to winning the boys competition.
Hiraoka has been in stunning form at the Cardrona halfpipe this winter, carding a succession of stellar scores which has continually left more experienced snowboarders struggling to match his form.
The youngster came second in the New Zealand Burton Open two weeks ago, and carded a run during qualifiers for the junior world championships which was judged the second-best score seen in a New Zealand halfpipe competition, bettered only by a winning effort from double-Olympic gold medallist Shaun White at the Burton Open last year.
The win was important for his halfpipe future, Hiraoka said.
"It's amazing to be world champion. I'm so happy."
Hiraoka held his nerve during gusty conditions at Cardrona yesterday to score 44.2, which left him comfortably clear of Australian Nathan Johnstone in second place with 42.2 points, while Manuel Pietropoli was third.
