The lanky American finished at 21-under 263, while Northern Irishman Rory McIlroy shot 67 for second place on 16-under.
Johnson has won at least once every year since he joined the PGA Tour in 2008. Only Tiger Woods (80) and Phil Mickelson (44) have won more often among players currently aged under 50.
"To get 20 wins out here is very difficult and to do it before I turn 35 is pretty incredible," the 34-year-old said in a greenside interview.
"It's tough enough to get one win out here. To win every year I've been on tour (has) been a lot of hard work and it's paying off right now.
"This is a big win for me and it gives me a lot of confidence for the rest of the year."
McIlroy cut the margin to two shots after three holes but could get no closer, with six back nine birdies being too little, too late.
Johnson finished 10 strokes clear of third-placed Englishmen Paul Casey and Ian Poulter plus Thailand's Kiradech Aphibarnrat.
Tiger Woods (69) tied for 10th, 13 shots behind Johnson, who started the final round four strokes ahead of McIlroy.
Johnson's second worldwide win in three weeks -- after the European Tour's Saudi International -- lifted him above compatriot Brooks Koepka into second place in the world rankings, just behind Englishman Justin Rose.
If there was a turning point on Sunday, it came at the fifth hole, where Johnson's drive ended up behind a tree.
He had luck on his side, though, and was afforded relief because a cart path interfered with his stance.
The free drop gave Johnson a clear shot at the green instead of having to chip out backwards, and he salvaged a par which helped settle the jitters.
On the next hole McIlroy found himself in a similar predicament but was denied relief and had to hack his ball out with a left-handed swing.
McIlroy's third shot found a pond. He bogeyed the hole, Johnson birdied it, and it was all but game, set and match.
"I definitely felt some nerves there at the beginning of the day but made some really good par saves early on and that gave me momentum," Johnson said.
"I knew I was swinging well. I just had to give myself chances and I turned it on on the back nine."
McIlroy, meanwhile, tipped his cap to the winner
"Some weeks a guy just plays better than you and that's exactly what happened this week," he said.
"He was very impressive all-round this week and deserved to win."
McIlroy has finished in the top five in all four of his starts this year.
"I can only control me, and I've played well. I've done what I wanted to do and needed to do," he said.
(Reporting by Andrew Both in Cary, North Carolina; Editing by Ken Ferris/ Ian Ransom/Nick Mulvenney)
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