The 34-year-old, who missed the cut at last week's Open, was one of four joint leaders into the final round but carded a six-under 66 for a 23 under total.

Englishman Tommy Fleetwood had four birdies and an eagle on his back nine in a 67 to tie for sixth, seven back.

Northern Ireland's Graeme McDowell finished in a share of 18th, 10 shots adrift, after a closing 69.

Also carding final-round 69s were Korean duo Whee Kim and Byeong-Hun An who tied for second place, three shots behind Johnson.

Keegan Bradley, without a PGA title since 2012 and six shots behind overnight, had holed his second shot from 151 yards for an eagle two at the par-four ninth, his fifth eagle of the week and birdied his final three holes to set the clubhouse target at 19 under.

Johnson was three shots ahead of the field after eight holes when the threat of thunderstorms caused a delay of nearly two hours at the Glen Abbey Golf Club in Oakville, Ontario.

He was only one clear of compatriot Bradley when he bogeyed the 12th, but birdied the next and another birdie at the 16th gave him a comfortable cushion.

Johnson finished in style with a 370-yard drive at the last and another birdie to secure his 19th career title, his 10th win in his last 43 starts.