SYLVANIA, July 19, 2013 (AFP) - Walshe finished with eight birdies with a pair of bogeys to complete the opening round at the Highland Meadows Golf Club course a shot ahead of former winner Paula Creamer, Lexi Thompson and Jessica Shepley.
World number one Park In-Bee, who has six LPGA Tour victories this season, is two strokes off Walshe's pace at four-under 67. She is tied for fifth place with Karine Icher, Kang Hae-Ji, Gerina Piller and Jacqui Concolino.
Walshe, who is ranked 90th in the world, is seeking her first career LPGA Tour win.
She began on the 10th tee and stumbled to a bogey on her opening hole -- but that was one of the few mistakes she would make.
"I've been hitting it probably the best I've hit it in a long time this year," said Walshe.
"I've been giving myself a lot of opportunities, and it's frustrating that I haven't been able to really go deep and make some putts.
"So I think ... the putting was key, and I think that's going to be key for me to really get the rounds going well."
The American parred the 11th, then drained a birdie putt at No. 12.
"I three putted the first hole and I was like, 'Here we go again,'" said Walshe.
"Then I one putted the next hole and got my confidence going. Made a lot of putts today, so that was pretty key.
"I think the difference for me today was definitely my putting.
"I had struggled on it last week and came out here and did a lot of work on it."
Defending champion Ryu So-Yeon carded a three-under 68. She shares 10th place with eight others.
Scores on Thursday after the opening round of the $1.3 million LPGA Marathon Classic (USA unless noted, par 71):
65 - Alison Walshe
66 - Lexi Thompson, Paula Creamer, Jessica Shepley (CAN)
67 - Jacqui Concolino, Gerina Piller, Karine Icher (FRA), Park In-Bee (KOR), Kang Hae-Ji (KOR)
68 - Natalie Gulbis, Chella Choi (KOR), Brittany Lang, Morgan Pressel, Mo Martin, Ryann O'Toole, Ayako Uehara (JPN), Ji Eun-Hee (KOR), Ryu So-Yeon (KOR)
69 - Wendy Ward, Lydia Ko (NZL), Chie Arimura (JPN), Pak Se-Ri (KOR), Sandra Changkija, Rebecca Lee-Bentham (CAN), Dewi Claire Schreefel (NED), Amy Yang (KOR), Jodi Ewart Sadoff (ENG), Beatriz Recari (ESP), Katie Futcher, Pak Jin-Young (KOR)