Horsfield took a one-shot lead into the final round and carded a closing 68 to finish 18 under par, a shot ahead of Belgium’s Thomas Detry.
Detry had recorded his ninth birdie of the day on the 17th to move into the lead for the first time, only to bogey the last after seeing his par putt from three feet catch the edge of the cup and stay out.
That left Horsfield needing to play the last two holes in one under par.
He birdied the 17th and after a cautious tee shot on the last, safely two-putted for par to complete a closing 68 and one-shot victory.
“It’s crazy. I can’t (describe my emotions),” Horsfield told Sky Sports. “On 17 we wanted to leave a good number and I hit a great shot there.”
England’s Chris Paisley, Welshman Oliver Farr and Sweden’s Alexander Bjork shared third place on 14 under par.
Spain’s Miguel Angel Jimenez, playing in a record 707th European Tour event, fired a final round even-par 72 to finish seven under for the tournament, 11 shots behind the winner.