Harwich were on top during the first half playing some neat football and creating chances, the early ones falling to striker jay Eggerichs.
There were two good headed efforts when defender Dan Keeble made sorties forward, his first attempt being well saved by Town goalkeeper Ben Self and the second going inches wide of the post. Another Shrimpers defender Tyler Sullivan saw his headed effort go close with half time approaching.
However the interval came with the match still goal less and goalkeeper Brad Cook having had a quiet time in the Shrimpers goal.
Diss were much more threatening after the restart and in the 58th minute Brad Cook went full length to touch a shot out for a corner and five minutes later made a brilliant stop turning a point blank effort over the crossbar.
Substitute Elliot Johnson had the Shrimpers only clear cut scoring chance of this half when from a cross he volleyed straight at goalkeeper Self.
For the second home game Cook was once more proving Harwich Man of the Match saving the Shrimpers again with his feet and then producing another super stop at the expense of another corner.
Diss were proving well organised and strong opponents and Cook was injured saving in a goalmouth skirmish.
This somewhat hampered the Shrimpers keeper and from a corner his defenders failed to give him any cover and the unmarked Charley Webb headed a vital 78th minute goal for Diss.
Harwich tried hard to save a point in the closing stages but Town made sure that they got all of their players behind the ball and a frustrated Shrimpers team were prevented from getting a clear sight of goal.
Harwich team: Brad Cook, Dan Keeble, Jack Wood, Tyler Sullivan, Reece Harte, Alfie Smith, Brandon Rusby, Jamie Beecham, Liam White, Luke Mayhew and Jay Eggerichs. Substitutes: Gareth Heath, Elliot Johnson, Harry Miller, Harvey Foreman and Archie Corner.