The 34-year-old entered the day four strokes clear of the field and never relinquished his firm grasp of the tournament. He made only one bogey and one double-bogey throughout the week while playing the other 70 holes at 24-under par.
Johnson's second win in Mexico in the past three years marks his 20th victory on the PGA Tour, and will give him a lifetime membership beginning in 2022. He's also won six WGC events, second most to only Tiger Woods.
McIlroy shot a 4-under 67 in Round 4 to finish at 16-under. He birdied six of his final eight holes but didn't come close to catching Johnson. The Northern Irishman has now finished inside the top five in each of his four events played this year.
Tiger shot 69 on Sunday and finished the tournament in a tie for 10th.