The $81,000 first-place check for Johnson gives him $412,915 for the season, setting the all-time single-season money record for the Nationwide Tour. He is the first player to ever break the $400,000 barrier.
'This is awesome,' said Johnson, who will play on a sponsor's exemption at the John Deere Classic on the PGA Tour next week. 'My expectations are a little high, especially after this week, but I'm just going to try to have fun. It'll be a good tournament to get my feet wet.'
Scott Gutschewski, one of the overnight co-leaders, finished alone in third place at 19-under-par 269 after a closing 3-under 69. Mark Hensby carded a 4-under 68 to wind up in fourth place at 17-under-par 271.
Johnson picked up birdies at the first and third for a quick start to move within one shot of Gutschewski. Gutschewski then birdied the fourth to reclaim a two-stroke advantage.
Johnson forged a tie for the lead at 18 under with birdies at the fifth and seventh. He thought his par save on the par-4 sixth was the key to his round.
'I hit my second shot in the left bunker and I was dead there,' said Johnson of his second shot to the sixth. 'It was straight downhill. I hit a pretty good shot but still had 15 feet for par. That putt was the big one.'
Gutschewski bogeyed the 10th to fall behind Johnson. Johnson, who won the Rheem Classic earlier this season, maintained a one-stroke lead as he and Gutschewski each birdied the par-5 12th.
Gage moved into contention with three straight birdies from the 10th, helping erase a front-nine bogey.
Gage, playing with Johnson, jumped into a share of the lead at minus-19 when he eagled the par-4 13th. However, Johnson took over first place by himself for good when he drained a birdie attempt on the 15th.
He briefly stretched his lead to two strokes when he came right back to birdie the 16th. Gage got within one stroke with a birdie at the 17th, but he could not birdie the last hole to force a playoff.
'I was confident, but I didn't know what to expect today,' said Johnson. 'On this golf course, with the pin placements and the wind and the firm greens, anything can happen. You can't be shooting for numbers. I was just trying to make birdies.'
Gage, despite the loss, was happy to see his neighbor and good friend pick up the winner's check.
'I had a feeling Zach was the guy to beat today,' said Gage, who closed with a 5-under 67. 'As well as he's been playing all year, I figured he's the guy. I played very well and I'm happy for Zach. He's a good friend of mine and he's played his butt off all year long. He's earned it.'
Gutschewski could only get back within two shots of Johnson as he birdied the two back nine par-5s, Nos. 12 and 17. He has now finished in the top-25 five straight weeks since his first event, a sponsor's invitation to the Omaha Classic.
Gutschewski has earned special temporary membership for the remainder of the season and climbed into the top-60 on the money list.
Joe Ogilvie, a two-time winner this season, finished alone in fifth place at 15-under-par 273 after a final round 67. Scott Gump and Franklin Langham shared sixth place one stroke further back at minus-14.