Harwich created a good first-half chance when Jack Elvestad broke through but the Hadleigh goalkeeper did well to come out and smother the ball.
It was nip and tuck during the first 45 minutes with Jevel Youngs pulling the strings in the Hadleigh midfield and Oscar Finbow (pictured) impressive in goal for Harwich with his safe handling.
The second half continued to be evenly contested but somewhat in contrast to the earlier play it was now Hadleigh who looked more dangerous in attack and Finbow pulled off a top-class save when he leapt to turn a goalbound shot over the crossbar.
Midway through the half, Hadleigh scored what was to prove the decisive goal with a high cross into the goalmouth that deceived Finbow and ended up in the back of the net.
Harwich made strenuous efforts to get back on terms but Hadleigh were formidable opponents and refused to give the Shrimpers a clear sight on goal.
With time running out, at last Harwich broke through but their goalscoring ace Dylan Crowley was thwarted when United defenders recovered and forced him away from the target.
For Harwich, it was their back four defenders who took the honours on this occasion, with right-back Taylor Francis taking their man-of-the-match award.