The tabulation of thousands of mail-in ballots that should have been posted on the registrar's website immediately after the polls closed appeared hours later.
"I appreciate Ms. Dunmore's service but it is time for new leadership," County Executive Officer Bill Luna said.
Dunmore was already under fire for similar complaints after the June vote. She was appointed registrar in 2004.
The registrar of voters office will be overseen by the county executive office until a replacement is appointed.