Ko, who began the day four shots off the lead of China’s Liu, worked her way through the field during the front nine but it was a stretch of four birdies in six holes in the middle of her closing nine which established her lead.
She was then required to wait close to 30 minutes before the title was officially hers as the closing two groups finished their rounds.
Ko gained access to the LPGA Tour by winning a jointly-sanctioned event on the Korean LPGA Tour in 2017 but it was her win at the 2018 Australian Open at Kooyonga Golf Club in Adelaide that brought her worldwide attention.
The rookie of the year in 2018 has since gone on to become the world number 9 even ahead of today’s win and is set to climb considerably higher.
"I thought really, can I win in the States?," responded Ko when asked what her first win in the US meant to her. "Last year I think too much, so I lose a little bit confidence. But my friends told me, You can do it. Also my manager and my parents, my caddie, all people say, 'you can'."
"So I got lots of confidence for this year. Really good for third win in the States, so I'm really happy."
Jessica Korda produced the equal best round of the day (64) to come from six shots off the lead to so nearly force a playoff, an encouraging performance after considerable disruption of late due to injury.
Australia’s Su Oh was the other player to produce a round of 64 and finished 18th and was the leading Australian.
Minjee Lee was 58th, Katherine Kirk and Hannah Green 71st and Sarah Jane Smith 78th.
Want video tips delivered straight to your inbox? Subscribe to iseekgolf.com newsletters.