The 32-year-old American closed with a 68 for a 14-under-par 270 total at Club de Golf Chapultepec, winning by a shot from Tommy Fleetwood after the Englishman holed a huge birdie putt at the last for a superb 66.
The success saw Johnson become just the second player after Tiger Woods to claim four or more WGC wins while he is the fifth golfer to win first time out as the top player on the planet.
“It means a lot because world No 1 is a tough spot to be in,” said the US Open champion, who toppled Australian Jason Day when winning last month’s Genesis Open in Los Angeles. “There’s a lot of pressure on you, but I came out and I played really well.
“I hit the ball great all week. The greens are tough to putt on and I didn’t feel like I putted my best, but I really hit the ball well and played just well enough I guess because I won by one.”
Fleetwood, a two-time winner in Scotland, including the 2013 Johnnie Walker Championship at Gleneagles, secured the biggest pay-day of his career.
The 26-year-old picked up over $1 million, opening up a gap of 965,676 points at the head of the Race to Dubai rankings in the process.
“It’s tough to beat the best player in the world, isn’t it?” said Fleetwood, who returned to top form when winning the Abu Dhabi HSBC Championship earlier this year.
“To be honest, I was kind of looking at trying to get third on my own at one stage, and then before you know it I saw the scores on the last green and I thought I might have a chance at winning it.
“But it was a really good putt on the last. It was a big one. It would have been nice to get in a play-off and see how we did, but overall it was a good day.”
On his WGC debut, Spaniard John Rahm shared the lead with three holes to play before taking back-to-back bogeys at the 16th and 17th.
He finished joint-third alongside Englishman Ross Fisher, who signed off with three birdies for a 65 that equalled the best score of the day.
“I didn’t play great every day and still had a chance to win the tournament,” said 22-year-old Rahm. “My first chance here on this golf course, first time I played this kind of event and to actually have a chance - it’s really nothing but positive.”
In his first event since January, having been forced to take a break due to a rib injury, Rory McIlroy was disappointed to finish joint-seventh after heading into the weekend with a two-shot lead.
“I needed to get off to a fast start and I didn’t,” said the world No 3 after carding two bogeys and two birdies in a closing 71.
“The course got a little trickier over the weekend. Maybe I just didn’t quite adjust to that.
“All in all, first week back, it’s okay. But, being two ahead going into the weekend, I’m obviously disappointed with the finish I had.”
The same certainly applied to Russell Knox after the Scottish No 1 crashed to a closing 81 to end up 70th in the 77-strong field.
The two-time PGA Tour winner saw the wheels come off as he carded a triple-bogey and three double-bogeys in six holes on his back nine.
Catriona Matthew has “things to work on” before resuming her 2017 campaign in the Kia Classic in California later this month.
The Scottish No 1 completed her first phase of the new season by finishing joint-30th behind Korea’s Inbee Park in the HSBC Women’s Champions event in Singapore.
It followed a missed cut in the Australian Women’s Open then Matthew fighting back from an opening 84 to finish in a tie for 45th in the Honda LPGA Thailand event.
“Good for the start of the year,” said the 47-year-old of her latest effort, having signed off with a 73 for six-under-par 282 total. “But I’ve got some things to work on during my couple of weeks off before going to the States.”
Park will be heading to California, where the season’s opening major, the ANA Inspiration, takes place just after the Kia Classic, in buoyant mood after the former world No 1 claimed a one-shot success at Sentosa Golf Club.
A closing eight-under 64 saw her finish on 19-under, one shot clear of her playing partner Ariya Jutanugarn (66). Sung Hyun Park (68) was third, a further shot back, while overnight leader Michelle Wie dropped back into a tie for fourth after a 72.
“My putting was amazing today, obviously,” said Park. “I couldn’t make any putts yesterday and obviously I made up for it today. “Pretty much everything I looked at, it wanted to drop in.”
Kylie Henry is sitting just outside the top 10 after the first round of the Joburg Ladies Open at Royal Johannesburg and Kensington Golf Club.
On a day when home player Monique Smith set the pace with a 67, the Scot carded a level-par 72 to lie in a share of 12th spot.
Wallace Booth signed off a three-event stint in Spain by tying for 18th position in the Swedish Golf Tour Winter Series Lumine Hills Open.
Now attached to Pitlochry, Booth carded scores of 67-72-75 for a one-under total, 10 behind Swede Oscar Lengden (64-67-73) at Lumine Golf Club.
Welshman Jack Davidson won the Spanish Amateur Championship after beating England’s Marco Penge by 4&3 in the 36-hole final at El Saler.
Davidson, conqueror of the leading qualifier, Balmore’s Christopher Maclean, in the first round, was two down after 14 holes in the morning before turning the tide his way following a strong start after lunch.
Englishman Will Enefer set a hot pace with an eight-under-par 64 in the opening round of the Algarve Pro Tour’s Quinta de Cima Classic.
The effort gave him a three-shot lead over compatriot Jamie Abbot as Conor O’Neil led the Scottish challenge with a 70, one better than Chris Doak.
Sweden’s Frida Kinhult is the new Spanish Women’s champion after claiming the title at Sherry Golf Jerez.
Kinhult, sister of European Tour player Marcus, beat Italian Alessia Nobilio at the 19th hole in a thrilling final.