ZANDVOORT, September 14, 2014 (AFP) - Casey, whose fiance Pollyanna Woodward gave birth to a baby boy on September 1, finished at 14 under par, one ahead of three-time champion Simon Dyson, who had a 67, while Wattel fell away with a closing 74.
Englishman Andy Sullivan meanwhile won a trip into space worth $100,000 for a hole-in-one on the 15th, although he admitted he was not certain to become an amateur astronaut.
Should he choose to accept the offer - his trip will be made in a Lynx Mark I aircraft next year
"I'm not sure yet," said Sullivan, who holed out with a nine iron on the 163-yard hole on his way to a closing 67. "I'll see what the missus says!"
Casey, who was ranked third in the world until injury laid him low, began the day four shots off the lead - just as he did before his last victory in the Irish Open in 2013 - but made the ideal start with four birdies in his first seven holes.
That set the tone for the day and, with Wattel fading fast, Casey moved clear and he never looked like being caught.
"I'm absolutely over the moon. First tournament as a dad, first win as a dad. This tournament has so much history, it's played in front of the best crowds we have in the world of golf, the turnout this week has been phenomenal," Casey told Sky Sports.
"The fact I threw in a few birdies early on was key, took the pressure off, and then it got tight at the end.
"It's a very difficult stretch coming in and I am very, very happy the way I balanced having fun on the golf course but being focused and got the job done."
Final scores at Zandvoort on Sunday in the KLM OPen counting towards the European PGA Tour:-
(Gbr & Irl unless stated, par 70):
266 - Paul Casey 68-70-62-66
267 - Simon Dyson 70-66-66-65
268 - Andy Sullivan 67-68-66-67
269 - Eddie Pepperell 69-68-66-66
270 - Romain Wattel (FRA) 67-65-64-74, Joost Luiten (NED) 65-70-67-68, Pablo Larrazabal (ESP) 68-62-75-65, Johan Carlsson (SWE) 70-67-70-63
271 - Richie Ramsay 69-65-65-72, Oliver Fisher 70-66-70-65
272 - Robert Jan Derksen (NED) 71-67-68-66, Mikko Ilonen (FIN) 69-66-70-67, Paul Waring 69-71-65-67, Raphael Jacquelin (FRA) 71-68-68-65, Tyrrell Hatton 68-67-72-65, Brooks Koepka (USA) 69-66-70-67, Jbe Kruger (RSA) 73-63-70-66, Edoardo Molinari (ITA) 66-66-71-69
273 - Richard Bland 70-70-67-66, Robert Rock 73-67-65-68, Gary Stal (FRA) 66-71-65-71