The 25-year-old birdied two of the last three holes to claim his first win on the European Tour as he finished one shot clear of compatriot Ross Fisher on an 18-under par total of 270.
Bradley Dredge of Wales, who led after each of the first three days and held a three-shot lead going into the final round, finished in a five-way tie for third a further shot back after a two-over par 74. "It's unbelievable and it's not sunk in yet," said Horsey who graduated to the European Tour after winning the Challenge Tour in 2008.
Fisher's round of 70, which ended with an eagle at the last, pushed him up to second while Horsey finished in emphatic fashion with birdies at the 16th and 18th.
"Coming down the last I didn't know what was going on. I just wanted to concentrate on what I was doing," said Horsey.
"This win is what I've been trying to get for over a year. I felt I was good enough coming out on Tour, but maybe I put a little bit too much pressure on myself." The biggest disappointment was undoubtedly felt by Dredge who appeared to be on course for his first victory in four years when he reached the turn without dropping a shot.
Dredge had only bogied once in the first 63 holes but his game began to collapse when he three-putted the tenth. His putter let him down again at the 12th and he then missed his pitch to the 16th. He managed a birdie on the last but it wasn't enough to challenge Horsey.
Spain's Pablo Larrazabal also had his chance to win, taking over at the top of the leaderboard when Dredge began dropping shots. Picking up five birdies in the first nine holes, the Spaniard turned in 31 but he came unstuck on the 16th where he double-bogied. He then dropped another shot at the par-three 17th.
Larrazabal said his mistake at the 16th had cost him the title. "I thought I had only three or four meters (of green) behind the flag, but I had eight. These things you do in practice on Tuesday and Wednesday.
"I lost the tournament there. It was a big mistake."
In Cromwell, Connecticut, Britain's Justin Rose is on target to claim his second PGA Tour win of the month after marching three strokes clear in the third round of the Travelers Championship on Saturday.
Rose carded a two-under 68 to finish on 16-under 194, three ahead of former British Open champion Ben Curtis (64).
American Vaughn Taylor (67) was third on 199 and seven players were involved in a logjam on 200 including Australian Matt Jones and US Ryder Cup captain Corey Pavin.
After notching 15 birdies in his first two rounds, Rose found it tougher going at TPC River Highlands on Saturday.
"It was just a day when nothing really went in on the greens," Rose said. "I guess the tale of the day was that I really made nothing. The longest putt I made was about - I think someone said four feet, 10 inches for par on 17.
"So I am really happy to have a day like that and still shoot 68 and maintain the lead." The Englishman, who seized his first PGA Tour victory at the Memorial Tournament in Ohio earlier this month, made birdies at the fifth and sixth but had to wait until the 15th for another.
He also came unstuck with a bogey at the par-five 13th after finding water off the tee.
"I really had only one loose swing all day, on the tee at 13," Rose said. "But after that shot I rallied well. I got up-and-down for par on 14, birdied 15 and hit it quite close there on 16. So bounced back from the only bogey of the day." Having gone winless in his first 161 PGA Tour events, Rose said his recent breakthrough would help him on Sunday.
"I'll sleep much easier tonight than I would have a month ago for sure," Rose said. "It's one less thing to think about, can I get it over the line. I've done that now, and I certainly would love to win my second tomorrow." Curtis's challenge came to life when he strung together five birdies on the back nine.
"I got off to a slower start today than I wanted," he said.
"But I just stayed patient and tried to keep hitting greens and make some putts. Luckily the last nine holes they started going in."
