Fox started the round tied for fourth and climbed to second before slipping back in the field in Ancaster, Ontario, with an even par final round which included six birdies and six bogeys.
Earlier, Fox was in contention for his first US PGA tour title, having a four-shot lead at one stage during the third round.
He finished the tournament at 10 under, six shots behind the winner Robert MacIntyre, of Scotland.
It was MacIntyre's first victory on the PGA Tour and will be memorable for many reasons, not least because his father Dougie was there every step of the way as a fill-in caddie.
They could celebrate together after the Scottish golfer shot 2-under-par 68 for a one-stroke tournament victory at Hamilton Golf & Country Club.
MacIntyre, a left-hander who earned his PGA Tour card via the DP World Tour Race to Dubai rankings, finished at 16-under 264.
That was one stroke better than Ben Griffin, who shot a bogey-free 65.
