Sac County was originally attached to Benton County. In 1851 the Legislature established the current boundaries of Sac and attached it to Wahkaw or Woodbury County. On January 22, 1853 it was attached to Polk County and on July 1, 1855 it was attached to Greene County for administrative reasons. A census was taken later that same year and 251 people were counted, enough to finally establish Sac County as an independent county. It was formally established by law in 1856 and named for the Sac Indian tribe, which was prominent in the area. The name means either "People of the Yellow Earth" or "red bank."

In November 1856 the citizens of Sac County petitioned the District Court to appoint a commission to locate the county seat. Sam H. Riddle, 7th District Judge, appointed two men to "within two months locate the Seat of Justice as near the geographical center as may be, having due regard for the present as well as the future population." Later that same month the county seat was established and named Sac City.

In the summer of 1860 a special election was held to determine the building of a courthouse. The election called for the raising of $10,000 - via 10 percent bonds - to be paid $1,000 annually and asked should a tax be levied. The bond issue passed and construction was begun in the fall by W. N. Neservery of Webster County.

By the end of the decade the citizens realized that they had built a courthouse that was too small to handle the demands of the county. This, coupled with the fact that the county government was changing from judges to supervisors prompted county and township officials to campaign for a new courthouse. Several elections failed to raise the needed support, and finally on September 6, 1869 the Board of Supervisors issued a resolution for the building of a county courthouse at a cost of $30,000 to be completed by October 1, 1873.

This courthouse burned after a prisoner started a fire in the county jail that spread to the courthouse. County offices were held in several buildings in Sac City.

A small battle for the county seat followed the burning of the courthouse. A group of businessmen from Wall Lake approached the Board of Supervisors and offered a "substantial sum of money towards the new courthouse." The citizens of Sac City then went to the Board with 1,745 names in support of Sac City. In the end Sac City won out, and a new courthouse was built at the cost of $30,000, $15,000 coming from insurance. It was completed in January 1890.

In the early 1970s two bond issues went to the voters for the building of a new courthouse. Due to the cost and the love of the old courthouse, both failed. The Board of Supervisors then hired architects to remodel the old courthouse and design a new building to house a courtroom, judges, clerk of court, and sheriff's office. This motion passed, because it was paid from the revenues from selling off the county farm and federal money. The county offices moved into the new addition in 1977.