Carri Wood, Michelle McGann, Wendy Doolan and Michelle Ellis share third place at 5-under par.
McGill opened on the back nine at Berkleigh and parred her first four holes before breaking into red figures at the 14th. She holed a wedge shot from 110 yards for an eagle and played even par over the rest of her front nine.
McGill drained a 20-foot birdie putt on the second hole and birdied two in a row at five and six. She flew the putting surface at the seventh but holed another shot from off the green, this time a chip-in birdie from 20 feet, taking her to 7-under and into the outright lead.
The 30-year-old pushed her drive into the right rough and punched her second over the green. McGill three-putted from the fringe for a bogey and fell into a tie for the first-round lead.
'Today is a result of what I've been working on,' said McGill, who played in Sunday's final pairing with Annika Sorenstam at the U.S. Women's Open. 'Anytime you have breaks like that, you are lucky, but I've been hitting it close.'
Ammaccapane also began on the second nine but her round was not as wild as McGill's. She drained a three-footer for birdie at No. 13 and followed two holes later with a 10-foot birdie. Ammaccapane closed her front nine with birdies at 16 and 18 and made the turn at 4-under 33.
Ammaccapane used great wedge play on her back nine to take her place on top of the leaderboard. At the par-5 fifth, Ammaccapane knocked her wedge shot to two feet to set up the easy birdie and closed her round with three-foot birdie putt at the ninth.
'It was actually nice and comfortable and easy to play out there today,' said Ammaccapane, a seven-time LPGA titlist who is winless in her last four years on tour. 'I was never between clubs and I was not in a whole lot of trouble. I think I could have had a lower score because I thought a few lipped out and I think I left a couple out there. Certainly I will take this as a first round.'
Moira Dunn, Susan Ginter-Brooker, Angela Stanford, Stefania Croce and Val Skinner share seventh place at minus-4.
Se Ri Pak, third on the LPGA Tour money list, opened with a 2-under-par 70 and is part of a group tied for 19th, four shots off the lead.
Karrie Webb, who won the Women's British Open two weeks ago, shot a 1-under 71 and shares 33rd.
Heather Daly-Donofrio, the 2001 Betsy King Classic winner, struggled in the opening round of her title defense. She shot a 4-over 76 and is part of group tied for 122nd.
Full-field scores from First Union Betsy King Classic