Park, who finished tied for third at last year's U.S. Women's Open, took home $900,000 of a $5 million purse.
The 17-year-old South Korean was poised to become the second amateur to win the biggest title in women's golf, joining 1967 champion Catherine Lacoste of France.
Park fired off her second consecutive five-under round to finish at 11 under for the event and earn a two-stroke win over Hye-jin Choi, the amateur who entered the day with a share of the lead.
Last year Choi was the low amateur, finishing 38th at CordeValle in California.
Park hit six birdies and one bogey in the fourth and final round to card an 11-under 277.
She leads Hye Jin Choi and Amy Yang (-8) by one stroke and Sung Hyun-Park by three strokes (-6).
Park prides herself on her ability to maintain her focus in the heat of competition.
However, Park tied for the lead with a 20-foot gain on the 15th.
Choi responded with a par on 17 and a birdie on 18 to claim sole possession of second place, thanks to Feng taking a triple bogey on the final hole to fall into a fifth-place tie with Carlota Ciganda and Jeongeun6 Lee.
"But I knew that, you know, on the last hole, I mean, when [Park] made the putt and people went like insane", said Feng, who led after the first, second and third rounds before stumbling Sunday.
Park won $900,000 USA for finishing first.
Choi's 279 was the best by an amateur in the Open, four shots better than the old mark by Grace Park in 1999.
McCarron finished 18 under to beat Langer and Brandt Jobe (72) by one shot, making up a six-shot deficit in the final round.
"Then on 18th I was like, "let's hit them little more closer" and I hit it to three feet and finally I made a putt".
"Coming to this week, I didn't have any expectation at all". "I just - I wanted to bring out my "A" game. I hit it better, I hit more fairways and I hit more greens", Lee said. "But I do not forget to stick with how I usually play because it was part of my practice and I stuck with it and I did it".
The 23-year-old is an LPGA rookie who previously dominated the Korea LPGA (KLPGA) Tour.
Feng shot a 1-under 71 to reach 9-under 207 at the president's Trump National Golf Club.
That put the Olympic bronze medal victor two clear of Korean trio Hye Jin Choi (69), Jeongeun Lee (69) and Amy Yang (71).
"She is definitely experienced player", Choi said through an interpreter. The weather really hasn't bothered me.
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A birdie at 17 and a tricky up-and-down on 18 saw her post the winning clubhouse score as her nearest rivals faltered behind her.
President Donald Trump and First Lady Melania Trump depart their viewing stand after the U.S. Women's Open on Sunday evening. He finished with a 73 that included only two birdies.
Inside the ropes, players battled puzzling greens and moody weather. She is now in third place with 95 points, with fellow South Korean Ryu So-yeon leading the way at 150 points, followed by Lexi Thompson with 105 points.