Reed entered the final round at Club de Golf Chapultepec trailing 54-hole leader Justin Thomas by one. He hung around by playing the first 15 holes at 2-under before three straight birdies propelled him into first.
The 29-year-old drilled a 17-footer for birdie on No. 17 to give him a two-shot edge and ultimately seal the victory. Reed bogeyed the last hole to finish the week at 18-under.
Reed's win can largely be credited to his red-hot putter. He had a total of 45 one-putts over 72 holes, the most by any PGA Tour winner over the last 30 years, according to the 15th Club's Justin Ray.
The victory marks Reed's eighth title on the PGA Tour and second World Golf Championship. He won this same tournament in 2014, when it was called the WGC-Cadillac Championship and played at Trump National Doral.
Here's how the top of the leaderboard finished in Mexico City:
|Place||Player||To Par||R4 Score|
|T3||Erik van Rooyen||-15||70|
Midway through the final round, DeChambeau looked to be in total control. He birdied four consecutive holes surrounding the turn and held a two-shot lead with only a few holes remaining. However, a sloppy par on the par-5 15th followed by a three-putt bogey on No. 17 proved to be costly in the end.
Thomas, who was the betting favorite heading into the final round, struggled off the tee en route to a 2-over 73. He hit only six of 14 fairways Sunday while carding four bogeys and a double-bogey.