The 25-year-old German, chasing her first LPGA title, fired seven birdies and an eagle in a bogey-free opening round at Columbia Edgewater Country Club in Portland, Oregon.
"Pretty flawless round," Mack said. "It seemed so easy out there today. Very nice start and that just kept me pretty calm I would say.
"Never really thought about it as a whole. Made some good putts. Just didn't make big mistakes or if I hit some bad shots I got myself out of it pretty good."
South Korean Jang Hyo-joon, Russian Nataliya Guseva, Germany's Caroline Masson and American Samantha Wagner shared second on 64 with Ecuador's Daniela Darquea and American Emma Talley on 65.
Mack, ranked 419th in the world, has missed the cut in 10 of her 13 LPGA starts this season. She missed 10 of 17 cuts last year as an LPGA rookie but managed her only top-10 finish, a share of third in last year's Great Lakes Bay Invitational.
Mack, a back-nine starter, birdied the par-5 10th and par-4 11th, then eagled the par-5 12th and birdied the 15th. After making the turn, she birdied the par-3 second and par-5 fifth before closing with birdies at the par-3 eighth and par-4 ninth.
"It feels so good," Mack said. "Feels like I've been working for almost years for this kind of stuff to happen. Just finally to see a result on the scorecard, not just in the game, on the course, but also seeing it written down is really nice."
Jang was out in the first group off the back nine and closed with birdies on four of her last five holes.
"I had bunch of 10-foot par putts that I made in the (first) nine, so I think that was huge part of today," she said.
Guseva birdied all four par-5 holes in her bogey-free round.
"I was just playing well. Really got into that momentum," Guseva said. "I just felt the confidence in myself and just hit a lot of good drives, made some putts.
"I was just really enjoying myself on the golf course. That was probably one of the best times for me on the LPGA."
Wagner made eight birdies in 12 holes in the middle of her bogey-free round.
"I was happy to see it all come together," she said.
