SOUTHPORT, July 13, 2014 (AFP) - The 31-year-old from Pasadena in California produced the shot of a blustery final round -- a three wood second at the 472-yard 18th that hit the pin and spun six feet away.
She holed the putt for an eagle three, posted a round of 72 and her one under par 287 total was good enough for a one shot victory over China's Feng Shanshan and Norway's Suzann Pettersen.
Martin had led by three shots at halfway but was three behind four-time major winner, South Korea's Park Inbee, going into the final round after a disappointing third round 77.
"The 18th was the greatest shot of my life," said the delighted new champion, who has won three time on the Futures Tour but never higher than third in three seasons on the LPGA circuit.
"I fell in love with Royal Birkdale from the first time I saw it."
Accuracy is 5'2" Martin's most potent weapon and it proved to be the key to her victory over a tight course with brutal rough. With winds gusting to 25mph, it was even harder on the last day.
One ahead at the start of the final round, Park, who won the 2007 US Women's Open, the first three majors last season and spent over a year as the world No.1, was two in front by the time she reached the turn in level par 36.
But a double-bogey at the tenth halted her march to a possible fifth major and another dropped shot at the 11th caused further grief and she fell behind Feng.
It then looked as though it would be a battle between the two Asians before Martin produced her miracle shot and moved one clear.
Feng, the 2012 LPGA champion, came to the 18th needing a birdie to tie but hit her second into a bunker and failed to get up and down.
Park was in the same situation but drove into deep rough and ran up a six so had to settle for a fourth place on one over par. Pettersen birdied the final two holes to earn a share of second with Feng on level par.
American Emma Talley won the Smyth Salver for the top amateur. The Kentucky 20-year-old closed with a great 73 for six over par and three shots better than her British rival, Georgia Hall, who shot a final round 74.
"Making the cut was great and winning the amateur prize was even better," said the player who holed the winning put for the US to win the Curtis Cup in June. But she won't be turning professional for at least two years.
Stacy Lewis, the American defending champion, shot 77 in the final round and was tied for 12th on five over par.
Leading final round scores on Sunday at the Women's British Open at Royal Birkdale (par 72):
287 - Mo Martin (USA) 69 69 77 72
288 - Feng Shanshan (CHN) 73 71 69 75, Suzann Pettersen (NOR) 72 73 68 75
289 - Park Inbee (KOR) 72 72 68 77
291 - Jessica Korda (USA) 72 72 73 74, Angela Stanford (USA) 74 72 70 75, Ji Eun-Hee (KOR) 74 70 71 76, Julieta Granada (PAR) 72 70 72 77
292 - Laura Davies (GBR) 75 72 72 73, Marina Alex (USA) 72 76 68 76, Ahn Sun Ju (KOR) 75 67 71 79
293 - Anna Nordqvist (SWE) 72 78 71 72, Azahara Munoz (ESP) 72 72 74 75, Gwladys Nocera (FRA) 73 70 73 77, Charley Hull (GBR) 73 76 66 78, Stacy Lewis (USA) 71 74 70 78
294 - Emma Talley (USA) 72 73 76 73, Ryu So Yeon (KOR) 71 70 74 79, Beatriz Recari (ESP) 74 67 74 79, Amelia Lewis (USA) 72 71 71 80
295 - Giulia Sergas (ITA) 76 73 72 74, Erina Hara (JPN) 73 74 73 75, Morgan Pressel (USA) 70 74 75 76, Paula Creamer (USA) 75 73 71 76, Lee Meena (KOR) 73 75 70 77, Amy Yang (KOR) 71 72 72 80
296 - Pornanong Phatlum (THA) 73 74 75 74, Rikako Morita (JPN) 75 75 68 78
297 - Georgia Hall (GBR) 73 72 78 74, Ayaka Watanabe (JPN) 76 72 75 74, Belen Mozo (ESP) 77 72 74 74, Mina Harigae (USA) 70 78 74 75, Carlota Ciganda (ESP) 74 75 73 75, Lee-Anne Pace (RSA) 75 73 72 77, Jenny Shin (KOR) 73 72 73 79, Shin Jiyai (KOR) 72 75 71 79, Lydia Ko (NZL) 72 76 69 80