The 23-year-old Park, a seven-time victor on the Korean LPGA Tour last year, was three shots back after tearing up the closing nine with six birdies for a 67 and a six-under total.
Park tied for third at last year's US Women's Open in her first appearance, holding the 36-hole lead before a pair of 74s saw her finish in third place, two strokes off the playoff. But in the final round she birdied the second, fifth, eighth, 12th, 15th and 17th holes to be two shots in front of Feng by the 18th.
Feng was frustrated by her putting until the par-five last hole.
Sung Hyun Park overcame a three-shot deficit and the heartbreak of a near-win in last year's event to capture the 2017 U.S. Women's Open.
South Koreans held the next six spots on the leaderboard, with 17-year-old world amateur number two Choi Hye-Jin and Amy Yang sharing second on 208 after third-round 70s.
Park's triumph at Trump National Golf Club may have surprised some at the course owned by U.S. President Donald Trump, but would have been no surprise to fans back home.
Park completed 72 holes at 11-under 277 for her first win on the LPGA Tour, which she joined this year. "I was just trying to get through the hole, maybe with a par".
Park Sung-hyun Wins US Women's Open to Claim Her 1st Major
McCarron finished 18 under to beat Langer and Brandt Jobe by one shot.
"She is definitely an experienced player", the teenager said.
Seventeen-year old Hye-Jin Choi of South Korea, who was trying to become the second amateur to win a Women's Open title, finished second after a 1-under 71 for a 9-under 279. But her downfall came on the par-3 16th, when her tee shot from 145 yards flared off short and right into the water. "Of course, I did have many winnings in other tournaments, but winning here at U.S. Open means so much more, and for that I am grateful and extremely happy". But she blasted out into the fairway, dropped her approach three feet from the cup then sank the putt to conclude a one-under 71 third round.
Yang, 27, said all her near misses at the championship would not get her down.
All week long, Sung Hyun Park was in the shadows like a lioness stalking her prey.
That third one came on the ridiculously hard 17th, extending her lead to three strokes and all but guaranteeing the LPGA rookie her first pro championship of any kind. They were speaking Chinese to me and rooting for me.
"I think one of the most hard things is to stay focused and keep that concentration level up because I know firsthand that once you lose it, your play is going to just go sideways". "Based on that good experience that I had last year, I think I was able to garner the championship this year". And Park has inserted herself into the conversation about the best players on the planet.
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