Horsey, who headed into the final round as joint leader along with defending champion Rhys Davies, made the perfect start with an ace at the 180 yard second and, despite following that up with bogeys on the third and fifth, he regained his composure with putts of eight and six feet on the seventh and eighth.
On 14 under he led South Africa's George Coetzee, who enjoyed his third blistering start of the week at Golf Du Palais Royal with four birdies in a flawless opening nine holes propelling him to second place on 12 under having started the day two shots off the pace.
Davies was three shots adrift of Horsey at the turn. The Welshman dropped a shot on the second but recovered well with birdies on the third and fifth to get himself back in contention.
Finland's Mikko Ilonen was two under for the day and ten under overall after nine holes, with Dutchman Joost Luiten and South Africa's Jaco van Zyl nine under through nine and eight respectively.
Denmark's Thomas Björn earlier signed for a course-record 62 to finish six under for the week.
"I played well - even with a bogey and a three putt," said the former Ryder Cup star. "It was just one of those days when I holed my fair share of putts and never made any big mistakes, hit 17 greens in regulation.
"The putts where rolling in and I was hitting it close a lot. On 16 my birdie putt was six inches, then six feet on 17 and then I made a really nice putt on the last for 62. It was one of those days, it was nice and relaxed."
Davies drew level with four consecutive birdies at the start of the back nine – a marvelous tee shot to within five feet at the 12th the highlight.
Horsey struck a potentially decisive blow at the penultimate hole.
His second shot from the fairway at the par five was within 12 feet, and Davies came up short of the green.
Horsey needed two putts, but Davies could not get up-and-down for birdie leaving his playing partner one ahead going up the last.