Reardon's announcement came at the end of his 10th State of the County address before business leaders in Everett.
He also said he supports an independent investigation, not only of himself and his staff, but of others in county government.
The executive alleged that since before his re-election in 2011, he has been the focus of "false and scurrilous accusations" leveled against him. It's part of "a concerted effort by groups that oppose" him that are intent on undermining his ability to lead, he said.
Reardon was investigated last year by the Washington State Patrol, and never charged, for allegations of misusing county money during an extramarital affair. He is the focus of a state Public Disclosure Commission investigation into using county resources on political campaigns, and he has been subject to repeated efforts by a Gold Bar blogger to recall him from office.
Reardon said that defending himself has taken a toll on his marriage, his ability to govern and has cost him "tens of thousands of dollars" in legal fees.
"Candidly, I don't know how much a family can take or should take ..." he told the crowd. "Enough is enough."
• Read the text of Reardon's resignation speech.
Reardon's announcement came the day after the County Council voted unanimously to remove his authority over the county's computers and records management system.
That step was taken in response to articles last week in The Herald, detailing evidence that members of Reardon's staff engaged in a campaign against his political rivals using public records requests, spoof email addresses and attack Web pages.
Many of those targeted were interviewed as part of the State Patrol investigation.
Reardon last week said that activity didn't happen at his direction, but he condoned the conduct, reasoning it was OK because he was told it occurred outside the office.
It'll be up to the Snohomish County Democratic Party to nominate three candidates to replace Reardon. A majority of the County Council will have to agree on the final choice after they receive the list of nominees.
The person appointed to be the next executive would serve until November 2014. Then it will go to the winner in an election for someone to fill out the remaining year of Reardon's term, which ends in 2015, county elections manager Garth Fell said.
No election is possible this year because Reardon's resignation is to take effect after filing week, which closes May 17.
The County Council will have 60 days after Reardon's resignation to appoint a successor. If the council is unable to reach a decision during that time, Gov. Jay Inslee will have 30 days to decide.
The state Republican Party called Reardon's decision to resign on May 31 a "final act of defiance" because it will extend by a year the term of whomever county Democrats nominate for the appointment.
"After everything he's put the voters through, it's time for Reardon to do the right thing and resign effectively immediately. Residents in Snohomish County deserve a chance to pick a replacement on Election Day 2013," the GOP press release said.
State Rep. Hans Dunshee, D-Snohomish, also said he found the timing of Reardon's resignation interesting because of the additional year it would give the appointee.
Before the day was out, people who watch politics closely were buzzing about who could be tapped to fill the opening.
Reardon's announcement came after he spent the better part of a half-hour delivering a speech about government and economic growth.
He began by stating that it has been an honor and privilege to serve for a decade in the community where he was born and raised. He said he was proud of accomplishments that should position the county to retain its place as a hub for aerospace jobs.
Then he acknowledged the controversies that have dogged him since November 2011. It was almost exactly a year ago that the County Council was urging Reardon to go on leave while he was being investigated by the State Patrol.
The Herald's editorial board endorsed Reardon two of the three times he ran for county executive. On Thursday, the editorial board wrote that Reardon's response to the recent revelations had created an "integrity vacuum."
County Councilman Dave Gossett, who attended Thursday morning's speech, said he was "totally surprised" by Reardon's resignation announcement.
Council Chairwoman Stephanie Wright also appeared caught off guard, but said she appreciated signs that Reardon wants to focus on collaboration during what are likely to be his final two months at the helm of county government.
• Read Wright's statement on the process to replace Reardon.
Regardless of what happens next, the council still wants an investigation into the records requests linked to Reardon's staff, but they are not sure what form that probe will take, she said.
Prosecuting Attorney Mark Roe was not at the gathering. In recent days he's been talking with police agencies about staging an independent, outside investigation of conduct by Reardon's staff.
Last year, he asked the State Patrol to investigate after a county social worker came forward to County Councilman Dave Somers as a "whistleblower" and reported that she had been traveling with Reardon for out-of-town rendezvous she believed were paid for using county money.
Patrol detectives became the focus of a formal complaint by one of Reardon's aides. Other people involved in the investigation, including witnesses who were approached by police, have been accused by Reardon and his backers of orchestrating a political smear.
Given that history, Roe said he's encountered "understandable reluctance by people who have too much to do already to delve into Snohomish County's laundry hamper."
But Reardon's resignation and his stated support for an investigation may change that, Roe said.
"Hopefully that will make for a compelling reason for an outside agency" to take the case, he said.
Reardon told the crowd Thursday that he planned to cooperate fully with the investigation he's requested.
On the advice of his attorneys, he declined to speak with patrol detectives last year.
Reardon's wife, Kate Reardon, a spokeswoman for the city of Everett, joined him after the speech. They left the Everett Golf and Country Club, hand in hand.
Reardon did not acknowledge reporters' questions except to say he had no comment.
Organized labor has typically been among Reardon's largest political supporters, but the biggest union representing county employees said that the county executive's troubles have made it difficult for members to work.
"You can notice the distraction when you're doing the type of thing that our organization does, which is represent the employees," said Chris Dugovich, president of Council 2, the AFSCME (American Federation of State, County and Municipal Employees) affiliate that represents about half of the county's union employees.
Dugovich said their members continued to do their jobs, regardless.
"They're pros, they're doing the same jobs that they've always done, even though the past few years have been difficult because of the economy. Those have been exacerbated by the public records requests, which have been coming allegedly from the executive's office," he said.
Some union members were targeted by the records requests. Other county employees spent hours pulling documents together.
Noah Haglund, 425-339-3465, firstname.lastname@example.org
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