The 2013-2014 World Cup winner clocked a combined time from her two runs of 1 minute 56.89 seconds to give her a 0.44 second lead over 2007 world champion Noelle Pikus-Pace of the USA.
In the first heat, the 25-year-old Briton, who is seeking to emulate the triumph of compatriot Amy Williams at Vancouver 2010, set a new track record with a time of 58.43sec.
“I was just eager to get two consistent runs,” said the British athlete at the end of the day’s session.
“I really enjoyed both of them. I let the sled run as much as I could -- it's a very enjoyable track.”
“I didn't even notice the crowds at the top, I was totally in my zone. I'm really pleased being in first position.”
Nikitina eyes home success
Meanwhile, Pikus-Pace said it would be tough to close the gap on Yarnold with two heats remaining.
“I'm happy to stay in the run for the title after two races though it's really hard to overtake the leader when you are almost half a second behind.”
Last year's European champion Elena Nikitina of Russia, who lay second after the first run, finished the day in third, 0.11 seconds behind Pikus-Pace, after a nervy second run.
“I'm not seriously upset with my mistakes in the second run,” she said. “Probably I would be even more nervous if I had done two flawless runs today.
And the Russian remained confident she could provide the home crowds with something to cheer in the concluding day’s runs.
“Tomorrow I'll try to correct them and to make it faster,” she added. “I'm currently at the peak of my form. I can do it.”
The podium places will be decided at the end of the third and fourth runs, with the times of all four runs added together to determine the winner.