Former Striker Jamie Morgan pounced to put the Seaford Town ahead on 15 minutes with an angled drive into the bottom corner.
United eased their way back into the game and equalised in 35 minute when the impressive Jack McLean had two efforts blocked on the goal line but Evan Archibald was on hand to blast home at the third attempt.
After the break Ben Davis was in on goal after a long punt upfield from replacement keeper Ray Collier, but the striker delayed his shot and the chance was gone.
Morgan had another effort well saved from Collier before Tom Morton fired over when well placed.
Another former United player Rafa Cuipinski surged into the box and set up Simon Stevens who finished well to make it 2-1 on 55 minutes.
Seaford furthered there lead when James Mckenzie found Morgan who made no mistake with the finish.
Morgan sealed victory Seaford when he converted another Rafa Cuipinski.