The 24-year-old bounced off two bogeys and a double bogey before three of the last five holes and scored a one-stroke victory over U17-year-old American Sophia Schubert.
Her 12th LPGA Tour win extends her lead as the most successful Canadian golfer in terms of wins.
On the deciding day, with just two shots separating Henderson from five third-placed under-15s, her par-71 was her highest score since cards of 64, 64, 68 in the first three days.
“It was definitely an interesting day…not the start I wanted,” Henderson said. “I remained pretty patient, as patient as I could under the circumstances.
“They say ‘majors are won on Sunday at nine’, so I was just trying to keep that attitude and I knew I would still be in it if I had a hard back.”
World No. 10 had a two-stroke lead on Sunday, but a slow start opened the door for a number of players to enter the fray.
Schubert took the lead in 12th, but Henderson squeezed back to level the American before the final hole.
Disaster struck when her tee hit the trees as Schubert made the green for the birdie, but the Canadian once again recovered superbly to avoid trouble. And with Schubert’s shot agonizingly rolling through the cup, Henderson was left with an 8-foot chance of avoiding the playoffs.
Henderson made no mistake by rolling home to claim her victory before her fellow competitors and sister Brittany doused her with champagne and water, which are caddies for her.
“To sit here for a two-time major championship is just an unreal feeling,” she told reporters.
“In 2016, winning my first major changed my life. My world rankings skyrocketed and I got a lot more attention from the fans and the media. championships and compete with the best in the world, it’s an amazing feeling.
“It’s been a long time and I got off to a fast start earlier this week. It was great to be at the top of the leaderboard in the Major. I was just trying to push that excitement to the limit.”
According to the LPGA, her $1 million payout to winners boosts her career earnings to $10,237,150, making her the 24th player in LPGA Tour history to break the $10 million mark.
“I want to cry tears of happiness”
After taking a short break from playing no LPGA tournament in May, the Canadian has since won two tournaments in four games after winning the ShopRite LPGA Classic in June.
“I worked a lot with my sister and also with my father who was my coach,” Henderson said.
“And the time spent at home with my mom really helped to give me some perspective and put my head in the right place so I could get back on tour and compete.
“I’m just really excited about what’s to come for the rest of the season,” she added.
For Schubert, the 2017 U.S. Women’s Amateur Champion, it was an agonizing end to what had been a hugely impressive performance in just her second career as a pro.
“I want to cry. I want to cry tears of happiness,” Schubert told reporters.
“I am proud of myself, proud of everyone who helped me get to this point. It wasn’t long, but I know I’ll be back so I’m really happy.
“It was what I always wanted. I knew that I could achieve this. There were some doubts, but I couldn’t have been in better company.”