Johnson is on top of the world after clinching first No.1 ranking
Dustin Johnson is the World No.1 for the first time after winning the Genesis Open.
The US Open champion took the lead in the second round and never relinquished it, ending the weekend 17 under par and winning by five shots at Riviera Country Club.
Johnson needed to win and hope Jason Day finished lower than a three-way tie for third to top the rankings - and the outgoing No.1 could only manage a tie for 64th after finishing two over par.
The leaders had to complete two rounds on Sunday, following storms on Friday.
Johnson took to the course on day four with a one-shot lead but was six strokes clear after the third round. The 32-year-old started with a birdie on the first and picked up further shots on the fifth, eighth and 12th before birdieing the last three holes for a seven-under 64.
That took him to 17 under for the tournament - equalling the 54-hole record.
The 72-hole record of 20 under was well within his sights after he gained shots on the first, second and sixth in his final round, but a first bogey in 50 holes dropped him back to 19 under at the turn.
He also recorded back-to-back bogeys on the 15th and 16th but the damage had already been done by then and he made sure of a PGA title for the 10th successive season with pars on the 17th and 18th.
Johnson said: "It was a great day. I played really well all day long and I'm very happy.
"It's obviously a big bonus to finally get to that No.1 spot. I'm very proud of myself and my whole team who have supported me. It's going to push me to work even harder."