SYLVANIA, 19 Jul 2015 (AFP) - Choi forced the playoff with overnight leader Jang with a five-under par 66 that included five birdies without a bogey.
"The first time win is hard, but the second and third is easier. Hopefully this is a turning point for me," said Choi.
Choi's father has caddied for her since she joined the LPGA Tour seven years ago. He said he wouldn't retire until she won, and now she has.
"I wanted to work with my father for my first win. So my father worked with me until now," she said.
Choi finished 72 holes on 14-under 270, tied with Jang -- a rookie also in search of a first LPGA title whose three-under 68 was marred by a double-bogey six at the par-four 11th.
Jang fought back gamely with three birdies after the setback, but the 23-year-old ultimately couldn't pull off a wire-to-wire win at Highland Meadows Golf Club.
Her approach at the first playoff hole, the par-five-18th, found the rough behind the green leading to a bogey, leaving Choi to claim the trophy with a par.
Defending champion and world number two Lydia Ko birdied four of her first eight holes and led by as many as two strokes midway through the round before falling back with a bogey at 13.
A birdie at 17 moved the 18-year-old New Zealand star into a tie at the top on 13-under, but both Jang and Choi birdied 17 to get to 14-under and Ko was unable to birdie the last to get into the playoff.
Ko finished with a 67 for 271, equal third with China's Feng Shanshan who also carded a 67.
South Koreans Kim Hyo-Joo and Baek Kyu-Jung and American Brittany Lang shared fifth place on 11-under 273.
World number one Park In-Bee moved within two strokes of the lead with birdies at the seventh and eighth, but didn't find another birdie in a round capped by a bogey at 18.
Park settled for a one-under 70 for 274, where she was joined by Spain's Azahara Munoz and American Angela Stanford.
Choi's victory -- after 28 prior top-10 finishes -- was foreshadowed by some excellent golf at last week's US Women's Open.
Choi fired the lowest third-round score in US Women's Open history, a six-under 64 in which her 29 on the front nine at Lancaster Country Club was a nine-hole record for a US Women's Open.
Leading final-round scores on Sunday in the US LPGA Marathon Classic (x-won at 1st playoff hole, USA unless noted, par-71):
270 - x-Chella Choi (KOR) 73-66-65-66, Jang Ha-Na (KOR) 66-67-69-68
271 - Feng Shanshan (CHN) 69-67-68-67, Lydia Ko (NZL) 71-66-67-67
273 - Kim Hyo-Joo (KOR) 71-68-67-67, Brittany Lang 68-71-66-68, Baek Kyu-Jung (KOR) 68-67-68-70
274 - Azahara Munoz (ESP) 73-68-68-65, Angela Stanford 68-68-69-69, Park In-Bee (KOR) 70-67-67-70
275 - Cristie Kerr 69-72-69-65, Haru Nomura (JPN) 71-69-67-68, Austin Ernst 70-68-66-71
276 - Kim Kaufman 71-68-71-66, Stacy Lewis 71-68-69-68, Ryu So-Yeon (KOR) 72-69-67-68, Kim Sei-Young (KOR) 68-70-69-69
277 - Lexi Thompson 72-67-71-67, Jaye Marie Green 69-70-68-70, Lee Mi-Hyang (KOR) 71-68-68-70, Jenny Shin (KOR) 68-73-66-70