The win was Johnson's first since returning from a six-month leave of absence from the tour and is his ninth PGA Tour title and his second WGC success.
The American's strong Sunday showing saw him finish one shot clear of J.B. Holmes, who led through the first three rounds, and two clear of Bubba Watson.
Johnson returned to the PGA Tour at the Farmers Insurance Open in early February after a break from the game to deal with some "personal challenges", and has looked in good form from the outset. He fell just short of victory at Riveria two weeks ago, losing out to James Hahn in a play-off, but would not be denied on this occasion.
The 30-year-old registered four birdies on the day, with a bogey on the seventh his lone blemish.
In contrast, Holmes struggled from the outset on the final day, bogeying the third, fifth and sixth to take the turn in 39, with his five-shot lead a thing of the past. A further bogey on 14 set him back even further, with his lone birdie at 16 not enough to trouble Johnson.
It initially looked like one of Bubba Watson or Adam Scott would profit from Holmes' decline.
Three birdies in the first ten holes saw Scott steamed up the leaderboard, but bogeys at the 11th and 18th stymied his charge and he was forced to settle for a tie for fourth with Henrik Stenson.
Watson was the mid-round leader after opening with four birdies in seven holes, but three bogeys in a four-hole stretch on the back nine saw him drop two back from Johnson.
World number one Rory McIlroy again failed to really hit his stride on Sunday, his round perhaps fittingly ended with a double-bogey after twice finding the water at 18. Despite this, he still posted a 71 to finish tied for ninth.