Rebels were looking to end their season on a high by retaining the cup which they lifted last season after victory over the same opponents, but Horsham’s Kery Kedze had other ideas as his 120th-minute winner saw them emerge 3-2 victors and lift the trophy.
The match, which had finished 2-2 after 90 minutes, had looked set for penalties until Kedze knocked home former Rebel Wayne Clarke’s low cross at the far post with virtually the last kick of the match.
It sparked wild celebrations from the visitors after a difficult season which had seen them relegated from the Ryman Premier Division, but left Rebels completely shell shocked at full-time.
It was made all the worse after Rebels had hit the woodwork three times in the game which would have seen them pull clear, although, to their credit Horsham also had a number of chances.
Another former Rebel, Charlie Farmer, nodded Horsham ahead five minutes before the break with a bullet header from Scot Harris’ free kick.
The lead lasted just five minutes, however, as, after Harris hauled down Jamie Brotherton in the box, Mike Huckett stepped up to the penalty spot, and although Michael Hunter saved his initial effort, the Worthing midfielder finished with the rebound.
The game picked up after the break and Ryan McBride should have done better with a free header within a minute of the restart, while Hunter did well to hold a fierce Budd drive.
Brotherton put Rebels ahead on 75 minutes as he superbly turned his man in the box to latch onto Budd’s pass and he finished across goal via the inside of the far post.
But three minutes later, Horsham were level as Dan Morris headed past Mansfield at the near post from Ryan Marriott’s cross.
Steve Metcalf came closest to winning it in stoppage time as he headed Huckett’s free kick onto the bar, while Luke Gedling blazed the follow up well over.
In the second half of extra time the referee made a crucial blunder as McBride put Josh Clack clean through on goal, but brought play back for a foul instead of playing the advantage.
With just minutes left the top of the post and bar saved Mansfield blushes as he appeared to leave a free kick that sailed over from Adam Hutchings, which the defender took from the halfway line.
But it was to be Horsham’s night as Kedze tapped in the late, late winner to spark wild Horsham celebrations.
Worthing manager Chris White said: “I’m very, very disappointed. I was pretty sure it was a game we were going to win and we controlled a lot of it, but just lacked that crucial bit of quality in the final third.”
WORTHING: Mansfield; Metcalf, Kennett, Woodburn, Hurley; Davies, Budd, Huckett, Lawley; Brotherton, McBride. Subs: Lawley, Clack (Brotherton 75), Lewis (Davies 107), Piper, Sparks.