In the early 20th century, it was a center of bituminous coal mining and in 1910 had a population of more than 25,000.

[2] The county, originally called Kishkekosh County after a famous chief of the Meskwaki,[3] was renamed for James Monroe, fifth President of the United States.

93.81% were non-Hispanic White, 0.25% were Black, 1.54% were Hispanic, 0.17% were Native American, 0.21% were Asian, 0.03% were Native Hawaiian or Pacific Islander and 3.99% were some other race or more than one race.

The racial makeup of the county was 98.40% White, 0.20% Black or African American, 0.36% Native American, 0.40% Asian, 0.12% from other races, and 0.51% from two or more races.

Of the 3,228 households, 30.50% had children under the age of 18 living with them, 56.20% were married couples living together, 8.60% had a female householder with no husband present, and 31.50% were non-families.

The population ranking of the following table is based on the 2020 census of Monroe County.