Wattel took a three-shot lead into the final round, but was unable to maintain the form that had seen him seize control of the tournament.
The Frenchman slumped down the leaderboard courtesy of a closing 74 and Casey, who recently became a father, took full advantage by shooting 66 to finish one shot clear of Simon Dyson at 14 under and secure his 13th European Tour title.
Another Englishman, Andy Sullivan, claimed third on 12 under and also earned the sensational prize of a trip to space courtesy of a hole-in-one at the 15th hole.
Wattel had to settle for a share of fifth on 10 under, while Richie Ramsay, second at the beginning of round four, also faded away to finish a shot further back.
Having surged into contention with a round of 62 on Saturday that equalled the course record, Casey carried on where he had left off with four birdies in the first seven holes on Sunday.
His excellent start was in stark contrast to the struggles of Wattel, who reached the turn in one over and mixed four further bogeys with a solitary birdie on the back nine.
Dyson did his best to apply late pressure, picking up five shots from the ninth to reach 13 under, while Sullivan also moved into contention courtesy of his ace.
However, a level-par back nine of 34 proved enough for Casey as he stood firm to claim a first win since last year's Irish Open.