The large lead allowed Hall to enjoy the rousing reception she received on her victory march up to the final hole, where a three-putt bogey was academic.
Hall got her nose in front thanks to another birdie on the 16th and Pornanong's double bogey on the 17th left the way clear for a memorable home victory.
Ahead of a tournament in Abu Dhabi previous year, Hall hit a drive of 579 yards on Yas Marina race track, believed to be the world's longest drive by a female golfer.
4 HE named Georgia in honour of Sir Nick Faldo's Masters win in 1996, having been born on the Friday of that victory in Augusta, Georgia.
"It was much harder than the first two days", Hall, now the number one player on the European Tour, sad.
But Hall chose the flawless moment to showcase her best golf and took the lead at exactly the right moment, sinking a ideal 20-foot putt on the sixteenth after Phatlum had found the green-side bunker.
Hall is the first British victor since Scotland's Catriona Matthew, also at Lytham in 2009, and joins Laura Davies, who has four, Alison Nicholas (1997 US Women's Open) and Karen Stupples (2004 British Open) as English winners of a women's major. Hall would be the first Englishwomen to win the Open in 14 years.
Pornanong Phatlum of Thailand plays a shot from a bunker on 12th green during day four of Ricoh Women's British Open at Royal Lytham & St. Annes on August 5, 2018 in Lytham St Annes, England.
To remain ahead, Phatlum birdied four of her first 10 holes and dropped her only shot of the day with a bogey on the par-three 12th.
Her round of five-under 67, which included six birdies, saw her finish at 17-under 271.
So Yeon Ryu (70 on Sunday) finished third, four shots back of Hall. "I'll be back here hitting long irons while those kids will be hitting wedges". "I was so happy".
"I was very confident going into this event from a year ago (equal third) and I love links golf and I'm playing in England, which is very rare". Hall averaged 28 putts per round and was seven-for-seven in sand saves. The Thai finished runner up for the fifth time in her career, which is also her career best in a major championship. I just had to stay calm.
And no countrywomen has ever accomplished the biggest feat in her sport at such a young age - with one of the great final rounds by any European golfer, to boot.
But instead of reflecting on the past, the former Oakmead College pupil hoped her triumph would inspire the next generation.
Thomas started the final round with a three-shot cushion and was never seriously challenged.
A birdie at No. 10 put him three shots behind, but Johnson bogeyed the last hole for a 64 and shared third with Thorbjorn Olesen of Denmark.
"I'm glad I finally played well around here, just in time to leave, " he said.