Kirk never trailed after heading into the final round at Annandale Golf Club with a one-shot advantage, and went on to win by the same margin - equalling the 22-under-par tournament record for four rounds in the process.
He was level with compatriot George McNeill with two holes to go but edged himself back in front at the 17th, hitting a bold 140-yard approach over water to within five feet of the pin for a straightforward birdie putt.
"People sometimes make more of it than what it is," said the 26-year-old from Knoxville, Tennessee.
"It was 140 yards and it was a nine iron, so I was aiming right at the pin and nowhere else. That was my only thought."
Kirk missed the cut when making his PGA Tour debut at this event in 2007.
"It's pretty amazing to think back to then, how far I've come from," he said. "My game isn't that much better than it was then, but just the comfort level that I have now to be able to go out for 18 today and feel 100% comfortable in my own skin."
McNeill also signed for a 68 to finish tied for second along with Tom Pernice Jr (67).
Bud Cauley (67), Jim Renner (68) and Hunter Haas (68) were a further shot back in a share of fourth.