At one point the 25-year-old was the outright leader on five under par, but a bogey at the 16th hole caused her to fall into a three-way share of first place on four-under 68.
That put Ewart Shadoff alongside Norway's Suzann Pettersen and South Korean Na Yeon Choi, whose compatriot Amy Yang was tied for fourth with Sweden's Anna Nordqvist.
Ewart Shadoff, known simply as Jodi Ewart until her January marriage to sports broadcaster Adam Shadoff, had six birdies in her round and made her move after the turn with four in a row from 11 to 14.
Defending champion Sun Young Yoo had a dismal day, the South Korean going from level par through 11 holes to five-over 77 by the end of her round after a rash of bogeys.
America's world number one Stacy Lewis stood one over par on 73, with number two Yani Tseng level par.
Ewart Shadoff is ranked 110th and began her golfing career while a member at Catterick in North Yorkshire, turning professional in 2010.
She said of her first round: "I just played really consistently, got into a really good birdie train in the middle of my round, which was nice.
"I was just hitting a lot of greens and making putts, so I'm happy with how I'm playing and excited to get out there tomorrow morning."