Dan Cronin takes over from Bob Schillerstrom, who is retiring after three terms.
Cronin previously served as a state senator.
He is also head of the DuPage County Republican Party.
Other newly-elected officials and board members also took the oath of office in an inauguration ceremony in Wheaton.
Onlookers included State's Attorney Joseph Birkett who was just appointed to be an appellate court judge. One of Cronin's first tasks will be to name a replacement for Birkett.