The 19-year-old Joh, of San Diego, rolled to a 6 and 5 win over the 14-year-old Kim, of Hilo, Hawaii, at Walking Stick Golf Club.
Joh was the equivalent of 11-under-par Sunday, needing only 31 holes to defeat Kim in what was a scheduled 36-hole final for the USGA's Robert F. Dwyer trophy.
'Honestly, I can't even really believe it yet,' Joh said. 'I'm just going to sit here and let it soak in...it doesn't even feel like I've won it yet.'
A soon-to-be UCLA sophomore, Joh placed an exclamation mark on her performance when she holed out for an eagle at the par-four 11th hole in the afternoon round -- the 29th hole of the final.
Joh held a 2-up lead after the first 18 holes, despite trailing 1-down after the front nine.
In that afternoon round, Joh won the par-4 10th with a par putt, then rolled in a 30-footer for birdie at the par-four 14th to move ahead. At the par-5 15th, Joh was conceded a birdie after chipping within 2 feet on her third shot.
Kim wasn't without highlights on Sunday. She chipped in twice for birdie and drove the green at the par-four ninth hole in the afternoon round.
'I knew that Kim was a fantastic player and I knew that just even-par was not going to beat her,' Joh said of her fellow competitor. 'I didn't expect to beat her at all. I mean, she would take it and run.
'She bombs it, she makes bomb putts, she makes bomb chips, and I really did feel like the underdog.'
Kim failed in her attempt to become the second-youngest women's champion behind fellow Hawaiian Michelle Wie, the 16-year-old star who was 13 when she won in 2003.