Postillion defeated Virginia Derby-Grimes, 2 & 1, in the semifinals, hours after downing defending champion Corey Weworski, 4 & 3, in the quarterfinals.
Lapointe breezed past Thuhashini Selvaratnam, 4 & 3, in the quarterfinals and then got by Robin Burke, 1-up in the semifinals at Shadow Hawk Golf Club.
Burke had upset two-time champion Carol Semple Thompson, one of the most- storied amateurs in history, on the 20th hole in the quarterfinals.
Postillion, who was the medalist in 1997, is trying to become the first medalist to win this tournament since Ellen Port in 2000. Postillion lost to Port in the 1996 final.
On Wednesday, Postillion, of Burr Ridge, Illinois, halved the final hole against Derby-Grimes.
Lapointe, who is from Canada, took advantage of a tired Burke to move into the final.
'You could tell by the shots that we were both tired,' said Lapointe. 'I was trying to keep up and get that second wind.'
Burke had actually evened the semifinal match through 16 holes, but she fell apart at the par-3 17th. She missed a par putt from seven feet, while Lapointe drained her putt from 15 feet. Burke then hit an errant shot on the par-5 18th, as her second shot went through the fairway, and she was forced to drop from a cart path into deep rough.
Burke's wedge shot landed on the green, 25 feet past the hole. She actually nearly made the putt, but the birdie attempt missed just inches from the hole. Lapointe's 20-foot putt came within a foot of the hole and Burke conceded the match.
The finalists will compete for the U.S. Womens Mid-Amateur Championship Trophy. A sterling Revere bowl, it was presented in 1987 by Mrs. Henri Prunaret, chairman of the USGA Women's Committee from 1959 through 1963. The champion retains custody of the trophy for the ensuing year.