The American, 32, closed with a round of 68 to finish 14-under par, one shot clear of England's Tommy Fleetwood.
Johnson only secured top spot from Jason Day two weeks ago, strolling to a five-shot victory
at California's Genesis Open.
Now he has a fourth WGC title -- more than anyone other than Tiger Woods (18) -- and becomes just the fifth player in history to win his first tournament as world No. 1.
The only other players to do so are Ian Woosnam, David Duval, Vijay Singh and Adam Scott.
"It means a lot because it's a tough spot to be in," said Johnson, alluding to the "pressure" of his new berth.
"I played just well enough I guess because I won by one."
The game's perennial nearly man is making up for lost time.
Without a major win until last year's US Open
, Johnson could easily have faltered here after lashing his first drive of the tournament out-of-bounds.
Instead, final-round birdies at the eighth and ninth holes gave him a four-shot lead that would be reduced but not relinquished.
There was plenty for Denmark's Thomas Bjorn to think about, looking ahead to Team Europe's challenge
at the 2018 Ryder Cup.
While Johnson held his nerve, taking home $1.6 million for his troubles, the rest of the top five was populated by Europeans.
Halfway leader Rory McIlroy was not among them, carding a final-round 71 to finish four shots back.
But both Fleetwood and promising young Spaniard Jon Rahm Rodríguez will be content with their showings, picking up the biggest paydays of their respective careers.
Fleetwood in particular could barely conceal his delight, even if he did botch his celebrations
after sinking a monster 40-foot putt on the eighteenth.
"That last putt was a big one wasn't it?!" said the 26-year-old. "It was just nice to have a chance going down the stretch and it just shows you're never quite out of it if you just hang in there."