Pornanong, the leader after the second and third rounds, started the day a shot ahead of Hall, opened her lead to two strokes after three straight birdies from No. 4, but was caught by her playing partner at No. 13 when Hall birdied.
Hall is the first English major victor since Karen Stupples won this event in 2004, and the fourth overall along with Laura Davies and Alison Nicholas.
But Hall chose the ideal moment to showcase her best golf and took the lead at exactly the right moment, sinking a flawless 20-foot putt on the sixteenth after Phatlum had found the green-side bunker.
Phatlum put her tee shot on 17 into the sand trap and three-putted the hole to all but clear the way for Hall's victory.
Hall was named Georgia in honour of the American state that hosts the Masters, which Faldo won two days after she was born, with a famous final-round 67 that mowed down Greg Norman.
At the age of 22, Hall - who finished tied for third at last year's Open - managed to record a first major title win and become just the fifith British victor of the tournament.
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"You know there are lots of little boys and girls watching the final round on TV and they've seen a young girl, at the top of her game, winning something quite sensational".
Brooke Henderson (74) of Smiths Falls, Ont., tied for 11th at six under par. The Kiwi produced a rollercoaster round of four bogeys and three birdies and was never able to make the charge she required to threaten the leaders.
"I always joked to myself, because I haven't won a tournament since I turned pro, that the first one I win would be a major".
"It is too good to be true", Hall said.
"It was my goal when I was nine-years old on the putting green, pretending to hole six-footers for the British Open. I was so happy". It was very close up until the final two holes, so I was just trying to hole the putts.
So Yeon Ryu (70 on Sunday) finished third, four shots back of Hall.