Mackenzie, of Yakima, Washington, guided the Americans to a big win over the Great Britain & Ireland team eight days ago. Monday, she was joined atop the leaderboard by Margaret Shirley of Roswell, Georgia, and Mi Jung Hur of Korea.
Playing close to home (the Curtis Cup was also held in Oregon) has helped the 23-year-old Mackenzie, a recent graduate of the University of Washington, over last two weeks.
'I've done well at home,' said the first-team All American. 'We hosted junior and college regionals and nationals and I felt really excited about coming home.'
Two of the co-leaders, Mackenzie and Shirley, double-bogeyed the 14th hole at Pumpkin Ridge's Witch Hollow Course -- a tough 393-yard 'par-four-and-a-half' reminiscent of the 13th and 15th at Augusta National.
For Shirley, her six at the 14th represented the only dropped shots in an otherwise spotless round that included four birdies.
Mackenzie's round was a little more up-and-down. Along with her double at the 14th, she carded six birdies and two bogeys. The one thing she did consistently was make putts, sinking four between 11 and 20 feet.
'I putted very well at the Curtis Cup too, and I have a lot more confidence,' said Mackenzie. 'It's the kind of mind set of being relaxed and trusting the line.'
Hur walked away with a par at the 14th, but had a double-bogey at the par-four first to go along with six birdies and two bogeys.
The leaders have a one-shot advantage over Mari Chun, In Kyung Kim, Kimberly Kim and Ya-Ni Tseng, who each carded rounds of one-under 70 on Monday.
Jenny Shin, who became the second-youngest winner of the Girls' Junior Championship last month, is tied for 23rd place after a round of two-over 73. Only six players have ever won both the Girls' Junior and U.S. Women's Amateur.
Stroke play continues on Tuesday, then it's five days of match play until a champion is crowned following the 36-hole final on Sunday.