Reardon, who was elected to a third term in 2011, made the unexpected announcement at a breakfast meeting in which he delivered the annual "state of the county" address. Reardon also called for an independent investigation of himself and his staff.
He later confirmed his resignation through a news release.
He said he has been the target of allegations since the last election campaign, and made the decision to resign due to the "emotional and financial toll these relentless attacks have taken on my wife, my family and me."
"My wife and I have been required to spend tens of thousands of dollars on legal fees in order to defend against false and scurrilous allegations," he said. "These false allegations have also taken a tremendous toll on my marriage and our family life."
"Enough is enough," he said.
During his term as the county's top executive official, Reardon has been accused of using taxpayer's money for out-of-town trysts with a woman who said she was Reardon's mistress, off and on, for the last six years.
A Washington State Patrol investigation into those accusations produced a 13,000-page investigative report that ultimately found only one questionable charge - an unaccounted $6 cab fare.
Afterward, the alleged mistress at the center of the affair, Snohomish County employee Tami Dutton, said the investigation didn't go far enough.
Then came disclosures about links between Reardon staffers and public records requests and other actions targeting the executive's political opponents.
The Snohomish County Council voted unanimously this week to remove Reardon's control of operation of the county's computers and other technology after those disclosures.
Reardon also was the target of two recall petitions that were ultimately were dismissed by the courts. He said Thursday he had learned that a third recall petition was in the works.
State Rep. Mike Hope, who ran against Reardon in the 2011 election, said he was "quite shocked" to hear about Reardon's resignation, but called it a "good step in the right direction for Snohomish County."
Hope said he believes the recent revelations about Reardon's staff "digging up dirt" on political opponents was the last straw that led to his decision.
"Over the last few days, some of the things I've read about with some of his staff, you know, working and digging up dirt and using it as a way to intimidate and harass people, and I think that's probably what led to this," Hope said.