The 45-year-old Lapointe earned a 1-up victory over Postillion to become the first foreign-born player to win the Women's Mid-Am and sixth Canadian to win a United States Golf Association event.
A four-time Canadian Amateur winner, Lapointe withdrew from this event last year with bruised ribs suffered in a car accident. She was playing in the championship for the fourth time.
'I am absolutely thrilled,' said Lapointe. 'It has been a goal of mine to win something outside of Canada. To win the Women's Mid-Amateur was a real goal of mine.'
Postillion, 42, was aiming to become the first medalist in five years to win this event. She fell behind early at Shadow Hawk Golf Club and came up short even after a late rally, finally conceding the match on the 18th hole.
The Burr Ridge, Illinois, native also lost in the 1996 final to Ellen Port.
'It was almost similar to that match,' said Postillion, comparing this final with her first nine years ago. 'I started off slow.'
Lapointe held a 4-up lead after 10 holes, getting off to a quick start with back-to-back wins on the first two holes as well as Nos. 6 and 7. She hit every fairway and five greens in regulation during those first seven holes.
But things changed quickly on the back nine, beginning at the 11th, where Postillion rolled in a 7-foot putt for birdie to pull within 3-down.
Lapointe then stumbled to three bogeys and one double bogey between holes 12 and 17, and her lead tumbled all the way to 1-up. But she didn't give up.
'I didn't hit the panic button,' admitted Lapointe. 'On the 17th tee, I told myself to forget what's happening. I had been sloppy the last few holes. But I said, 'I'm 1-up with two to play. A lot of people would be thrilled to be in my position.''
After the duo halved the 17th, Postillion hit into a fairway bunker off the tee at the par-5 18th and then duffed her approach shot just 40 yards after escaping the sand.
Lapointe landed 15 feet from the hole on her approach shot and putted within inches before Postillion, looking at a long putt, conceded the match.
'I never got it going,' said Postillion.