Visitors Hadleigh United were the first to threaten but Harwich goalkeeper Jack Lines came out to make a good block in a one-on-one situation.
With 10 minutes gone, Alfie Canning gave Harwich the lead with a good finish but 10 minutes later Hadleigh came back to equalise, when again Lines was forced to advance but this time was beaten by a very well-taken shot that powered over his head and into the roof of the net.
Within another minute, Harwich had a golden chance when Liam White was brought down but from his penalty kick goalkeeper Chris Warren dived to save.
Undeterred, Harwich were back in front when Harvey Smith sped through to give Warren no chance to save his shot.
It was end to end and Lines pulled off another good save, this time with his feet, to thwart United's powerful striker.
Ten minutes into the second half, Harwich increased their lead with a smart goal from Ewan Livingston (pictured) and he figured again when he was brought down and this time substitute Jack Elvestad, a penalty specialist, sent Warren the wrong way with his spot kick.
Three goals down, Hadleigh then staged a comeback by netting twice to reduce the Harwich lead to 4-3.
The Shrimpers needed a goal to halt the Hadleigh charge and it came with good work down the right flank from Jaidan Smith and he was rewarded when he found the net.
Hadleigh were not done yet and again they reduced the margin to a slender goal but Harwich steadied themselves to claim victory at the final whistle.
Harwich team: Jack Lines, Alex Pryke, Oscar Warnes, Sol Griffiths, Archie Corner, Joe Simmons, Ewan Livingston, Liam White, Alfie Canning, Idreess Namisi and Harvey Smith. Substitutes: Jaidan Smith, Batuhan Macit, Rory Breen and Jack Elvestad.