The Englishwoman reeled in long-time leader Pornanong Phatlum in a gripping final-round duel at Royal Lytham to win the Women's British Open for her first major title on Sunday.
Hall, who had held her nerve throughout the back nine, negotiated a bogey at the last to go with her previous six birdies on the day, having held a three-shot lead.
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 fourth English victor of the tournament.
"I first played this tournament in 2012 and, since then, it's become the one that I want to win", said the 2011 US Women's Open victor, who added a second major at the 2017 ANA Inspiration. I've gotta go out there tomorrow and try to play my best and maybe be a little bit more aggressive than I had been going.
Playing in the final group with Rory McIlroy, the 25-year-old Thomas never let anyone closer than two shots of the lead.
Pornanong, also seeking her first major and LPGA title, double-bogeyed No. 17 to leave Hall with a three-shot lead down the last.
Home favorite Georgia Hall (68) was in the tie for second place along with, who was leading by two strokes on 11 under when she lost her ball in a gorse bush at No. 17 and wound up with a double-bogey 6.
Sports Minister Tracey Crouch praised Hall's performance, saying: "Congratulations to Georgia Hall on winning her first major in what was an incredible performance".
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Another dropped shot came at the 17th, but birdies at 15, 16 and 18 when Hall sent an approach within six feet of the flag saw her produce a strong finish.
In 2017, Hall returned to the Women's British Open to finish in tied-third, a high she matched in the World Ladies Championship. "There were so many people behind me that I feel that was the reason I had good shots".
Hall was not at the top of her game, and was happy to remain only one shot behind.
Phatlum shot a final-round 70 but was no match for Hall down the stretch.
Her bogey on the last hole was her first of the round, and she dropped just three shots in the whole tournament - an astonishing achievement on such a hard links course. I don't think it has all sunk in for me. That regained a two-shot lead for Pornanong, who had also birdied the par-three fifth hole.
Hall contested an epic final-day battle alongside Thailand's Pornanong Phatlum - the pair racing away from the rest of the field to virtually set up a matchplay situation. "I'm in a pretty good position".
Hall played safe in three-putting from distance in front of Royal Lytham's storied clubhouse and celebrated her first win on the LPGA Tour. A short sideway escape and an impossible approach, ended up with a triple bogey on the par-four third and another bunker encounter on the fourth put Ryu back far enough that despite some fine play she wasn't able to make up the lost ground.