Kim claimed the title in her first and only appearance at the event, rolling to a 5 and 4 victory over the 2002 champion Park at BanBury Golf Club.
A former member of the Korean National Team who speaks little English, Kim was surprised when Park, also born in Korea, gave her a hug after conceding the 14th hole.
Apparently, the champion believed she and Park were to play a 36-hole final.
'I turned to (chaperone Kim Campbell) and she said, 'Only 18 holes.' I said, 'Really?' I was very happy.'
After going 2-up over the first two holes, Kim doubled her lead to 4-up with a 20-foot birdie putt at the par-4 seventh hole. Park, who struggled with her control all day, put her drive on No. 7 into the deep rough.
'The advantage of Koreans is that they hit [their] driver straight,' Park said. 'I wasn't one of them today.'
Park birdied the ninth hole to get within 3-up, but Kim came right back with a birdie at the next to regain her 4-up lead. Another birdie for Kim at the 11th extended that margin to 5-up, where it remained until the 14th hole.
On No. 14, Park knocked a 5-wood into the water hazard, and after her opponent rolled a 42-foot putt close to the cup, the 2002 champion conceded the hole and the match.
'I wasn't nervous, but I wanted this so bad,' said Park. 'I wanted to say sorry to my parents because I didn't win. They've supported me so much.
'There were 155 players who cried after their last round. I was one of them. I just cried late.'
Kim will play in the Women's Amateur -- for which she had already qualified -- in early August. She will be looking to become the first player to win both tournaments in the same year.
'I'm really happy,' said Kim. 'Excited. One more step in my career.'
The U.S. Girls' Junior was established in 1949 and is open to any female golfer who has not reached her 18th birthday by the close of the competition and has a USGA Handicap Index that does not exceed 18.4. The Girls' Junior is one of 13 national championships conducted annually by the USGA, 10 of which are strictly for amateurs.