Johnson tuned up for the July 14-17 US Open by carding birdies on two of the last three holes for a one-shot victory in Memphis, Tennessee, while Rory McIlroy squandered a chance at winning with an errant tee shot on his final hole.
"I tried to just play my game today," Johnson said. "I'm hitting the ball well. I was just trying to play my game and get as many looks at birdies as I could. And, you know, I did that."
Johnson was injured trying to lift his jet ski off a trailer and, following a return at last week's Memorial tournament where he tied for 19th, he fired a four-under 66 in the final round at TPC Southwind to finish at nine-under 271, one clear of fellow American John Merrick (68).
He claimed his sixth PGA Tour title, moved back to 10th in the world and became just the second player after Tiger Woods to win in each of his first five years out of college.
"It feels really good. Especially having so much time off and coming back," said Johnson. "Last week helped a lot competing and getting back in the groove of things and then I came out this week and I just played well all week."
Ryan Palmer (66), Chad Campbell (68), Australian Nick O'Hern (69) and US Ryder Cup captain Davis Love III (69) shared third at seven-under 273.
It was a rollercoaster day for Northern Ireland's world number two McIlroy, who arrived at the 18th hole tied for the lead only to send his three-wood tee shot into a water hazard and end up with a double-bogey.
"I was missing that three-wood left all day," said McIlroy, who finished tied for seventh. "To be honest, I wasn't really that comfortable with it, but I knew it was the shot that I needed to play and I didn't hit the way I wanted to."
Johnson started the final round one shot back of overnight leaders Merrick, O'Hern and Love III but fired a laser approach inside four feet on the opening hole and converted the birdie to join the lead.
Back-to-back birdies on the sixth and seventh holes, the first via an impressive 17-foot putt and the next coming from yet another close wedge shot, ensured the 27-year-old stayed on top of the leaderboard.
But when he failed to get up and down from the greenside bunker on the ninth hole and posted bogey, his momentum stalled and a handful of challengers leapfrogged him on the board.
Six consecutive pars kept the American in the mix before he made his move, just as those ahead of him faltered.
Johnson birdied the par-five 16th to join the lead pack at eight-under and as McIlroy and Campbell sent their tee shots on the 18th into the water, Johnson drained an eight-foot birdie on the 17th to take the outright lead at nine-under.
A routine par on the last proved good enough but it left Merrick and O'Hern, playing in the final group, a chance to tie with birdies.
O'Hern dunked his tee shot into the water while Merrick found deep rough but advanced the ball to just short of the green and almost chipped in to force a playoff.