Day was never a threat and finished the week in 64th position but given the trials and tribulations he has experienced with his health and injuries, even during his reign at the top, he has done well to hold on as longs as he has.
He had also held the top spot for four other weeks earlier.
FINAL ROUND HIGHLIGHTS GENESIS OPEN
Johnson began the final round of the Genesis Open at the Riviera Country Club five ahead of Web.Com Tour graduate Wesley Bryan but with three birdies in his opening six holes the lead over the field quickly stretched to eight.
Over the closing stages, perhaps the margin of the lead or perhaps the length of the days involved in the weather delayed event took their toll but he would bogey the 9th, 15th and 16th to finish five shots ahead of the fast finishing Thomas Pieters and Scott Brown.
Johnson becomes the 20th golfer to claim the top spot since world rankings were introduced in the mid 1980’s, that list quite a walk down memory lane.
Bernhard Langer, Seve Ballesteros, Greg Norman, Nick Faldo, Ian Woosnam, Fred Couples, Nick Price, Tom Lehman, Tiger Woods, Nick Price, Ernie Els, David Duval, Vijay Singh, Lee Westwood, Luke Donald, Rory McIlroy, Adam Scott, Jordan Spieth, Jason Day and now Johnson have been or are the official world number ones.
The winner has now also won in each of his 10 PGA Tour seasons since winning for the first time in 2008 when successful at the Turning Stone Resort event.
Amongst those victories have been three World Golf Championship titles and his only major to date, the 2016 US Open.
"It gives me a lot of confidence and will push me to work harder and try and get even better,” said Johnson in the aftermath of his victory.
"I didn't finish the last 10 holes the way I would have liked to but you know I had a pretty big lead with 10 holes to go so I was kind of on cruise control but would have still liked to have played a little better over the last 10 holes."
Belgian Thomas Pieters powered home over the final 18 holes on Sunday afternoon with a round of 63 to jump 23 places and share runner-up spot with American, Brown.
Pieters, a three time European Tour winner and currently 47th in the world ranking, will move higher to around 35th.
As an added bonus, as a part timer on the PGA Tour at this stage, the significant cheque he earns for his runner-up finish ($US598,000) will lift his chances of gaining full PGA Tour status in the next few weeks, especially given he has another start at the World Golf Championship event in Mexico in 10 days’ time.
Brown was chasing his second PGA Tour victory nearly four years after his first but this would be his second biggest cheque and his best PGA Tour finish since that win.
Justin Rose tied for 4th with talented rookie Wesley Bryan, Kevin Na and Charley Hoffman.
Adam Scott continues his overall good record at Riviera when he tied for 11th but suggested afterwards that he was not happy with his game over the final 36 holes.
“Hopefully a little time on the range should sort it out,” he said afterwards as he looks ahead to defending his title next week at the Honda Classic in Florida.
Cameron Smith was 28th, Cameron Percy 49th, and Jason Day 64th