The South African shot a four-under-par 67 in the final round on Sunday to finish on 19-under 265 for the week.
She had set the course record at Ottawa Hunt and Golf Club on Thursday with a first-round 62, but entered Sunday trailing South Korean rookies Hye-Jin Choi and Na Rin An by a stroke.
But Reto birdied the first, third, sixth, eighth and ninth holes to shoot ahead to 20 under for the tournament, playing the back nine in one over which was still enough of a cushion for her to see the win through.
Her tap-in par at the par-five 18th secured the milestone victory.
"I was so nervous, oh my gosh," Reto said on the CBS broadcast.
"Coming to the last hole, I told myself, 'OK, just calm your nerves.'"
The 32-year-old has played on the LPGA Tour since 2014, with her previous best finish being third place at the Yokohama Tire LPGA Classic in her rookie year.
"I'm really happy, and it's been such a long time," Reto said. "I was like, 'I hope one day I'll win.' You know? But it came true. A dream come true for sure."
Korda, the world No.3, made a late push after playing the front nine in even par. She birdied the 11th before holing out for an eagle at the par-four 12th, jumping to 17 under at the time.
But after a bogey and two more birdies, Korda couldn't hole a birdie at either of the last two holes to tie Reto at 19 under. She also carded a 67 and tied for second with Choi (69).
Lydia Ko, of New Zealand, ranked fourth in the world, rolled in birdies at each of her last four holes to shoot an eight-under 63, tied for the low round of the day. She finished alone in fourth at 17 under.
Rising Australian Steph Kyriacou finished in joint-17th after recording a final round 70 to finish at 10 under, while compatriot Hannah Green was a shot behind after also firing a final-round 70.
10°C