The American collegiate champion led from the first flight and sped away from a fast-finishing Parchment, who clocked a season best 13.08. Commonwealth Games champion Andrew Riley was third in 13.35.
McLeod become only the second Jamaican to run under 13 seconds for the event.
"At the NCAA it was not the perfect time but I knew with the perfect wind I could run that 12 (seconds) ... so it's just a scintillating feeling," McLeod told Reuters.
"This is just an extra boost somewhat, I'm really happy, I'm elated and ready for Beijing," he added in reference to the Aug. 22-30 world championships in the Chinese capital.
McLeod also said he would compete in Diamond League meetings in Europe ahead of Beijing after foregoing the remainder of his collegiate eligibility and turning professional.
Defending world silver medallist Warren Weir remained on course for a spot to Beijing in the 200m after placing third in his semi-final in 20.26, behind Julian Forte who registered a personal best of 20.04.
Nickel Ashmeade, who placed third in Friday's 100m final easily won the other semi-final in 20.23.
With world champion Shelly Ann Fraser-Pryce opting out of the women's equivalent, her training partner Elaine Thompson laid down the marker for Sunday's final with 22.56.
Beijing Olympics 100m silver medallist Sherone Simpson, who was fourth in the 100 on Friday, won her 200 semi-final in 22.62 ahead of two time world and Olympic champion Veronica Campbell-Brown with 22.75.
Jamaica will have two representatives in the women's 800m for the first time after Natoya Goule and Semoya Campbell both secured career best marks, by breaking the two-minute barrier.
(Editing by Greg Stutchbury)
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