Whitton were the last team to defeat Harwich and so this match had an extra edge for the Shrimpers to reverse that result.
Harwich made a good start and took the lead in the 12th minute. Goalkeeper Brad Cook's long kick was headed on by Joe Knight for fellow striker Louie Bloom to flick on to Alfie Canning, who was brought down in the penalty box. Luke Mayhew then confidently coverted the resultant spot kick.
The Shrimpers were on top in these early stages and Knight put Bloom through for his shot to be blocked on the goal-line.
There were 23 minutes on the clock when Harwich scored their second goal. United's goalkeeper Kade Ivatt managed to push out Alfie Eldridge's shot, but only into the path of the oncoming Knight who smashed it into the net.
Harwich continued to be in the ascendancy and Canning's 25-yard drive whistled just over the crossbar.
Just before half time, Harwich won a second penalty, this time for a handball, but Ivatt saved to thwart Mayhew.
Whitton defended resolutely upon the restart to frustrate Harwich and then came more into the game.
Midway through the second half, the Harwich defence was called into action with Cook making an excellent save to tip the ball to safety and two minutes later Eldridge making a goal-line clearance.

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With 15 minutes remaining, Harwich scored their third goal to put the result beyond any doubt. Youngster Oakley McNair, making his first start, initiated the move with a smart pass to Mayhew who crossed for Knight to stab home.
It seemed that Harwich would keep a clean sheet but in the final minutes Fin Watts' effort was diverted by a defender past Cook.
Nevertheless, this was a very good result for Harwich, one which extends their unbeaten run to 11 matches - their best such streak since the 2018/19 campaign.
The win keeps them comfortably in third spot in the table, six points behind second-placed Stanway Pegasus but with two games in hand. They now require only three more points to confirm a play-off spot, although that second place will be the longer-term goal.
Harwich team: Brad Cook, Oakley McNair, Reece Harte, Jack Wood, Dan Keeble, Alfie Eldridge, Gareth Heath, Alfie Canning, Luke Mayhew, Louie Bloom and Joe Knight. Substitutes: Alfie Smith, Luke Spurling, Tyler Sullivan, Liam White (all used) and Keelan Milburn (unused).