The long-hitting American surrendered a big lead on the back nine before securing a one-stroke victory over Englishman Tommy Fleetwood at Club de Golf Chapultepec.
Johnson finished at 14-under 270 for his second consecutive victory, after winning the Genesis Open two weeks ago.
That five-stroke victory at Riviera in California vaulted Johnson to the number one world ranking for the first time. If he felt the pressure in Mexico, it hardly showed.
"It means a lot," Johnson, who shot 68, said in a greenside interview when asked how it felt to win in his debut as number one.
"It’s a tough spot to be in. There’s a lot of pressures on you. I came out and played really well."
Johnson's victory was a reward for patience, after he started the tournament by hitting his drive out-of-bounds at the very first hole.
He also avoided getting frustrated by the bumpy greens, unlike many of his rivals, and worked his way back to build a four-stroke advantage over young Spaniard Jon Rahm with eight holes remaining on Sunday, only to give up his lead with back-to-back bogeys.
Johnson again remained calm, avoided any further dropped shots in a three-under-par round and tapped in at the last from a few inches.
Fleetwood (66) birdied the last to claim second place on 13-under. Rahm faded late to tie for third with Englishman Ross Fisher on 12-under.
Overnight leader Justin Thomas faltered with a 72 to finish three strokes behind, equal fifth.
Expected challenges from Rory McIlroy and Phil Mickelson never left the launch pad.
McIlroy and Mickelson started the day two strokes from the lead, carding 71 to tie for seventh on 10-under.
Johnson has now won 14 times on the PGA Tour, including four victories in World Golf Championships events.
"I felt like I hit the ball great all week," Johnson said.
"The greens, they’re tough to putt on. I didn’t putt my best but I really hit the ball well and played just well enough I guess, because I’ve won by one."
(Reporting by Andrew Both in Cary, North Carolina; Editing by Toby Davis/Gene Cherry)
source: news.abs-cbn.com
