[2] The county was formed in 1851 and named for Major Frederick Mills of Burlington, Iowa, who was killed at the Battle of Churubusco during the Mexican–American War.

[3] Mills County is included in the Omaha–Council Bluffs, NE–IA Metropolitan Statistical Area.

[4] The future county's first permanent settlement was Rushville, founded in 1846 by persecuted members of the Church of Jesus Christ of Latter-day Saints as they were being driven out of Nauvoo, Illinois.

A nearby settlement, also founded by the Mormon settlers, was called Coonsville after Dr. Liberius Coons, one of the first arrivals.

That settlement continued after the Mormons moved on; its name was changed to Glenwood in 1853.

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

He was succeeded by James Hardy who served as the first elected sheriff of the county and assumed the office on August 31, 1851.

He and his 11 full-time deputies patrol approximately 447 square miles in the county.

It has backed the Republican in all but five elections in its history, its inaugural election in 1852 prior to the founding of the Republican Party, in 1912 when former Republican turned Progressive Theodore Roosevelt caused a split in the vote, allowing Democrat Woodrow Wilson to take the county with a sub-40% plurality, in the two landslide victories for Franklin D. Roosevelt in 1932 and 1936, and for Lyndon B. Johnson in his 1964 landslide, who even then barely took Mills County by only 39 votes and less than 1%.

The board of supervisors is elected according to Plan One of the 1984 Code of Iowa, Chapter 331.208.

Josh England is the current Mills County, IA sheriff.