Yarnold, 25, emulated compatriot and 2010 champion Amy Williams, sliding quickest throughout the four-heat competition.
She took victory by 0.97 seconds, ahead of American Noelle Pikus-Pace with Russian Elena Nikitina third.
With the Sanki track riding faster, a confident Yarnold built on her opening day advantage by setting a venue record with her first slide on Friday and despite the odd bump, eased to victory on her final run.
British women have won skeleton medals at every Games since their event was introduced to the Olympic programme in 2002 in Salt Lake City.
(Reporting by Justin Palmer, editing by Peter Rutherford)
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