"It was always my dream to play on the LPGA. Just being here was a dream come true," said Ko, who started the day one back of leader Lexi Thompson. "For these amazing things to be happening is unbelievable but it also motivates me to work harder."
Britain's Charley Hull (69) and South Korea's Chun In-gee (70) finished a stroke back. Thailand's Ariya Jutanugarn (71) led late but bogeyed the final three holes to finish two strokes off the pace.
The season's first major looked headed for a playoff when Jutanugarn's bogey at the par-three 17th, where she pulled her tee shot badly, set up a three-way tie for the lead.
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But Ko, who was coming off a win at last week's Kia Classic, would secure her 12th LPGA Tour victory by striking a precise wedge from 88 yards to set up a one-foot birdie putt.
Following her triumph, Ko took the traditional leap into Poppie's Pond next to the 18th green along with her mother, sister, caddie and caddie's fiance.
The South Korean-born Ko was bogey free for the final round and made several crucial putts to stay in the hunt, including pair of long birdie putts on the front nine and a solid par-save on the 13th to stay two shots back of Jutanugarn.
"This been a long week," said Ko. "I’m proud I was able to make those clutch putts, especially on the back nine."
Jutanugarn, 20, made a red-hot start to her quest for a maiden LPGA Tour win as she carded five birdies and one bogey over her opening 11 holes.
But Jutanugarn was undone by a disastrous finish that saw her three-putt at 16, send her tee shot at 17 into a bunker before finding water on 18.
American Thompson (73) finished three shots back of Ko in fifth place while defending champion Brittany Lincicome (74) finished 10 shots off the pace.
(Reporting by Frank Pingue in Toronto; Additional reporting by Andrew Both in Cary, North Carolina)
