The 25-year-old, who led the event after the two opening runs, maintained her consistency in the third and fourth rounds to clock a combined time of 3 minute 52.89 seconds. She finished a full 0.97 seconds ahead of second-placed Noelle Pikus-Pace of the USA.
In her third heat, Yarnold, who emulated the triumph of her compatriot Amy Williams four years ago in Vancouver, set a new track record with a time of 57.91 seconds.
2013 European champion Elena Nikitina of Russia was third fastest overall, claiming the bronze with a combined time of 3 minutes 54.30 seconds.
Valentine’s Day delight
Over the first three runs, Yarnold had built up such a cushion that she went into the final run knowing that only a bad mistake would cost her the title,
“The fourth run I was totally relaxed and went out and enjoyed it," said the British champion.
“It was a bit of a messy run but I'm thrilled to get here after working so hard for the past five years, which makes it all worth it.
Yarnold had her family looking on to witness her finest moment on the track, capping what was a truly memorable 14 February for the Briton.
“It's also lovely as it's Valentine's Day, so it has something romantic about it as well,” she enthused.
“My mum and dad and my sisters are here and I couldn't have done it without them or the Team GB skeleton team. I'm Olympic champion.”