Schmidhofer started the day with a lead of 90 points in the chase for the small globe awarded to winners of individual disciples, AFP reported.
She went off third and, even though she finished 11th, she clinched her first title when Siebenhofer, following two slots later, almost fell and, though she stayed upright, was slower down the piste and ended 19th.
Puchner edged German Viktoria Rebensburg by 0.03sec and Swiss skier Corinne Suter by 0.08sec for her second career victory.
On a good day for her country, Schmidhofer ended a 12-year women's downhill drought by winning Austria's first globe in the discipline since Renate Goetschl in 2007.
That was the year Schmidhofer won two junior world titles, in super-G and giant slalom. But the skier, who turns 30 on Friday, was slow to deliver on that youthful promise at the highest level, until she won the super-G world title at Saint-Moritz two years ago.
This season she has finished on the World Cup downhill podium four times winning twice, in consecutive races in Lake Louise.
Puchner's only previous victory came in Saint Moritz in 2016.
Mikaela Shiffrin opted not to compete on Wednesday, her 24th birthday. The American, already assured of the overall and slalom titles, can add the super-G title today and the giant slalom title on Saturday. She leads the standings in both disciplines.