The 23-year-old replaced Amy Williams on the British team for the fifth World Cup leg in Koenigssee last week after a disappointing campaign so far saw the Olympic champion record a best of just sixth.
Yarnold, who won silver at the Junior World Championships last year, didn’t disgrace herself finishing 14th overall on debut however few would have expected what happened in Switzerland.
She produced a storming first run of one minute 11.93 seconds for the overall lead – 0.25 seconds ahead of team-mate and Olympic silver medallist Shelley Rudman, who won in Koenigssee last time out.
Germany’s Katharina Heinz was third just 0.04 further back but well clear of team-mate and reigning world champion Marion Thees who was nearly a second behind Yarnold with a run of 1:12.89.
And, with heavy snow falling and conditions worsening in St Moritz, the second schedule run was cancelled handing Yarnold, who won this season’s Intercontinental Cup, a somewhat unlikely victory.
Rudman, who has found her form of late, finished second to move up to the same place in the overall rankings ahead of Heinz, who ended third, and just 23 points behind Thees, who stayed fourth.
The FIBT Skeleton World Cup will now move on to Canada and Whistler and Calgary for the final two rounds before the World Championships in Lake Placid.