Trump abruptly fired James Comey last week (May 9).
"I'm both honored and humbled to be appointed to this position," Clarke said on Wednesday.
But not everybody was happy for him.
"Our country deserves better," said Milwaukee County Executive Chris Abele.
Abele said Clarke has engaged in "gratuitous name calling, insults of other public officials, questioning judges' sentences. In general the focus is on what you should be angry about, not what to like."
The conservative sheriff is a staunch pro-Trump supporter and a frequent guest on CNN and Fox News.
Clarke, 60, leaves his position as sheriff of Milwaukee County amid controversy after an inmate was found dead in his jail.
38-year-old Terrill Thomas died last year after he was deprived of water for seven days. Clarke did not face charges in the inmate's death because a grand jury found that he was not directly responsible.