The Englishman found himself three shots behind overnight leader Fox after eight holes on Sunday but showed his mettle on the back nine, clinching a two-shot victory with a winning score of 13 under par courtesy of a closing 68.
Horsfield's third victory, following his wins at the Hero Open and Celtic Classic in 2020, comes just two weeks into his return from a three-month injury lay-off.
And the 25-year-old, who had his partner on the bag this week, was emotional as he completed a fourth round which contained four birdies and a single bogey.
Fox finished in a tie for second on 11 under par alongside German Yannik Paul, with Chase Hanna and Oliver Bekker another shot further back.
Horsfield started the day one stroke off the lead but joined Fox at the summit early in the final round after the overnight leader dropped a shot at the second hole.
But Fox bounced back in style, holing from 27 feet on the third green to return to 11 under.
A stunning approach from Fox at the fourth set up a five-foot birdie, and he made it three in a row from nine feet on the fifth to go three clear.
The 35-year-old came agonisingly close to adding a fourth straight birdie from the fringe at the sixth but narrowly missed.
After six straight pars, Horsfield picked up his first birdie of the day at the seventh to close the gap to two, but a nine-foot birdie at the eighth restored Fox's three-shot advantage.
Fox had to settle for a two-stroke lead at the turn after an errant tee-shot on the ninth led to a bogey there, but it could have been so much worse, with the New Zealander producing one of the shots of the week to give himself a ten-foot par chance, but the putt narrowly slid by.
Horsfield, however, holed his 15-foot par putt on the ninth to keep up the pressure on Fox.
And Horsfield closed the gap to a single shot after making another birdie from around 15 feet at the tenth.
The pendulum swung back Fox's way at the 11th, though, as Horsfield made a bogey after getting into trouble off the tee.
But Horsfield trickled in his close-range birdie putt at the short 12th to reduce the gap to one once more.
And a bogey from Fox at the 13th saw him hand Horsfield - who rescued a par despite a wayward tee-shot - a share of the lead on 12 under.
Both men parred the 14th and 15th before Horsfield grabbed the outright lead at the 16th after Fox made another bogey to slip back to 11 under.
And Horsfield took a one-shot lead into the final hole after he sunk a vital eight-foot putt to match Fox's birdie at the 17th.
A par at the last proved enough to seal Horsfield's third victory, and with Fox dropping a shot there, his winning margin was two strokes.
He said: "I was trying not to cry while I was over that little tap-in (at the 18th). Having Issi on the bag, it's been an amazing week.
"Mick (Seaborn, his regular caddie) is not here but I wish he was. I said yesterday I wanted to do it for him and I was able to do it for him. I'm just so, so happy.
"I definitely thought about Mick quite a lot. Especially on the back nine, I pulled a few clubs that he definitely would have wanted me to hit.
"Issi has been amazing. I've been so happy on the golf course. It's just like golf's sort of got in the way this week, until a little bit there on the back nine.
"I asked her coming down 18 'are you nervous' and she said yeah.
"But we've had an amazing week and I'm just so happy that she was here to enjoy this.
"I feel like I've been playing really good for a while but haven't really been able to put four rounds together.
"This week I played solid all week and next week the US PGA, I'm really looking forward to it and definitely the summer ahead."