Spaniard Garcia, 32, finished the tournament - which took place over three rounds due to stormy weather on Friday - on 18 under for his second victory of 2012.
He birdied seven of the first nine holes then another four in a row on the back nine, despite a rain delay halfway through them.
American Jonathan Moore signed for a 61 to finish second, a stroke ahead of Thailand's Thongchai Jaidee (65).
Thaworn, 45, shot a 65 to tie for fifth at 11 under and win the overall Asian Tour title for a second time. His closest Merit challenger, Marcus Fraser of Australia, closed with a 72.
"I didn’t feel any pressure because there was only one competitor – Marcus Fraser. I put in a lot of effort and focus to win this year so I’m very happy that I finished as number one," said Thaworn.
"I never thought I could win the Order of Merit at this age. I just enjoy playing week by week and meeting my friends on Tour. I’m quite happy with that."
He earns a five-year exemption on the Asian Tour and also gains exemptions into the Open Championship and WGC-Cadillac Championship next season.