 The original county consisted of the portion of the contemporary Mille Lacs County east of the west branch of the Rum River and two townships now part of Isanti County, adjacent to Mille Lacs County.In 1858 the 12 townships forming the contemporary southern 10 townships of Mille Lacs County and the two northwestern townships in Isanti County were organized apart from Benton and Mille Lacs Counties to form Monroe County, leaving the northern "Square Top-knot" as Mille Lacs County.In 1860, Monroe and Mille Lacs Counties merged.