Known for his often jaw-dropping rhetoric and incendiary conservative commentary, Clarke had served as one of the country's more well-known sheriffs since 2002, but he raised his national profile as a regular surrogate for the Trump campaign.
Clarke did not provide any reasons for his departure in a resignation letter obtained by CNN
and did not respond to requests for comment.
Thursday afternoon Clarke posted a picture of himself on Twitter "(i)n Nashville for National Fraternal Order of Police convention," leaving no hint of his imminent resignation.
Over the weekend the President -- seemingly out of no where and in the midst of Hurricane Harvey -- endorsed Clarke's recent book, tweeting, "A great book by a great guy, highly recommended!"
CNN's KFile reported in May that Clarke plagiarized portions
of his 2013 master's thesis on US security, though he vigorously denies the allegations.
Critics of Clarke point to what they term a sordid history
of his management of the county's jail, but the past few months have also been marked by questions over whether he would be tapped for a formal role in the Trump administration.
In May, Clarke announced on a local Wisconsin radio show that he was accepting an appointment as Assistant Secretary in the Office of Partnerships and Programs at Homeland Security -- yet days turned into weeks without a word of confirmation from DHS.
Finally, DHS said in June that he was "no longer being considered" for a position within the agency, and a DHS official confirmed to CNN Thursday that he is still not up for a job there.
And while Clarke makes regular TV appearances on Fox News, a source with knowledge of the situation told CNN that there have been no discussions, at least thus far, about hiring Clarke.