But Jutanugarn managed to steady the ship to force the contest to sudden death as Kim failed to get up and down from a greenside bunker on the second hole (No.18). Confidence gave way to fear.
With the win, Ariya will climb to No 2 in the world later this week as she sets her sights on returning to the top spot she held for two weeks past year.
Henderson shot a 1-over 73 in the first round on Thursday.
As is the custom for LPGA winners, Ariya Jutanugarn found herself doused with water after the final hole.
From up seven strokes with nine holes to play on Sunday of the U.S. Women's Open, to down one shot in a two-hole playoff, television announcers took to discussing how a potential failure might derail her entire career.
They returned to the 18th hole for the third time Sunday.
Despite losing her big lead, Jutanugarn says she wasn't feeling nervous going to the two-hole playoff. "It was unusual, the front nine". Karrie Webb has a fight to make the cut, having played seven holes and is four-over-par for the championship. Jutanugarn had some wiggle room. She hits it a long way, but her short game is impeccable.
Jutanugarn needed a break, and she got one at 16.
"He's like, 'Come on".
"Just keep thinking about those two 3-footers I missed", said DeChambeau, who was at 14-under 202. "I'm like, 'Yes.' He said, 'OK, then we have to do it.' OK, so I hit". But Kim made bogey - her first of the day - on 18 while Jutanugarn made par again to send the format to sudden death. The 22-year-old Korean dropped a 50-foot birdie putt at the 12th. "I want to win a tournament when she plays good and I play good".
She won the Lotte Championship this year and has two previous top 10 finishes in the US Women's Open.
"She rolls the ball beautifully".
"She's a spectacular player, obviously", Smith said. She birdied the first extra hole, No. 14, while Jutanugarn made a par.
Sixty feet from the flagstick with a pond looming not far beyond, she blasted a fearless shot that trickled within a foot of the cup.
She opened the day in a three-way tie atop the leaderboard with Jutanugarn and Korean Jeongeun Lee. Kim In-kyung trails them at No. 7. Then I actually put a Callaway 3-wood in last week which has been really good. "I just haven't been putting for many birdies".
South Korea's Kim Hyo-joo was alone in third after shooting a four-under 68 while another shot behind was Kim Ji-hyun (70) at five-under 211.
There were lots of smiles after the long, wet week concluded with a long, wacky day.
Ariya has eight LPGA victories in the past two years, including a major at the 2016 Women's British Open.
There's a charm in the childlike pranks she enjoys playing on her older sister, Moriya.
"It's kind of fun seeing that", she said. "But no matter what, you're going to learn from them". "You can do everything'". She finished as the runner-up.
However Ariya won't be returning home just yet because she still has plenty of events to come in the United States and Europe.
"I told myself to just go out and have fun and be really committed, and I did", Ariya said. It was just like the front nine felt really nice.