The American finished five shots clear of Northern Ireland's Rory McIlroy on a score of 21-under-par at the Chapultepec Golf Club.

Johnson led by four going into the final day and held onto his advantage to end the competition at the top of the leaderboard.

McIlroy threatened to overhaul Johnson as a blistering front few holes saw him move to within two shots of the American player.

The four-time major winner eventually carded a four-under 67 to finish on 16-under-par.

England's Paul Casey and Ian Poulter were joint third on 11-under, while Tiger Woods of the United States made three birdies in his final round to end tied for 10th.

"It clicked and I hit it really well this week," Johnson said. 

"It means a lot, to get 20 wins out here is very difficult. 

"To do it before I turn 35 is pretty incredible.

"This is a big win for me. 

"It gives me a lot of confidence for the rest of the year. 

"I feel like the game is in good form right now."