The 23-year-old, looking to make just her second cut in her first year on the LPGA Tour, shot a bogey-free seven-under 64 to open up a comfortable lead over her nearest rival Carlota Ciganda.
"I always felt like I wasn't too far away really. It was just like one bad round ... just couldn't really make any putts or whatever," said Masson. "I felt like, 'OK, there's a good round in me. I just got to be patient.' And yeah, I think that's what I did."
Spain's Ciganda had three birdies - all on par threes - even though she only hit four of 13 fairways in her opening round.
"I think the main defence was my putting today," said Ciganda. "That's really good for me. So I think that was the key today because I've been hitting good shots, good driver. So I'm just going to try to keep playing good and let's see if I can make putts."
World number one Inbee Park was among five players at 67, three shots off Masson's early pace.
Taylor Coleman, 16, shot 68 after winning through a special qualifier reserved for high-school golfers. She ended the day ahead of world number two Stacey Lewis, who had a one-over 71.
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