But now Clarke has resigned his position as sheriff.
"Pursuant to Wisconsin Statute 17.01, this communication is submitted as the notice of my resignation as Sheriff of Milwaukee County commencing August 31, 2017, at 11:59 p.m.", stated Clarke's resignation letter.
In an email to The Milwaukee Journal Sentinel Clarke said that "only someone with a political agenda would say this is plagiarism".
The sheriff could not immediately be reached for comment.
He was a vocal supporter of then-candidate Donald Trump during the 2016 presidential campaign, even speaking at the Republican National Convention. Previously, he suggested that he was being considered for a post at Trump's Department of Homeland Security - but that ended up unconfirmed. Tammy Baldwin, a Democrat, but he repeatedly rejected their overtures. Following a CNN report in June that accused him of plagiarizing his master's thesis, the sheriff withdrew his name from consideration for the position. He recently published a memoir, "Cop Under Fire".
Clarke was one of the few African-Americans to speak at the Republican National Convention a year ago.
Under Clarke, the Milwaukee County Jail has become the focus of an investigation following the deaths of four inmates in a six month period, including the death of a man who died from dehydration after the water in his jail cell was turned off for six days.