The county seat and most populous city is Junction City. The county is named in honor of Governor John W. Geary.
For many millennia, the Great Plains of North America was inhabited by nomadic Native Americans. From the 16th century to 18th century, the Kingdom of France claimed ownership of large parts of North America. In 1762, after the French and Indian War, France secretly ceded New France to Spain, per the Treaty of Fontainebleau.
In 1802, Spain returned most of the land to France, but keeping title to about 7,500 square miles. In 1803, most of the land for modern day Kansas was acquired by the United States from France as part of the 828,000 square mile Louisiana Purchase for 2.83 cents per acre.
In 1854, the Kansas Territory was organized, than in 1861 Kansas became the 34th U.S. state. Geary County was formed on August 30, 1855 as an original county from open, free territory. It was among the first 33 counties established by the territory government.
Geary County was originally named Davis County in 1855 after Jefferson Davis. In 1862 and 1864 after Davis became president of the confederacy, attempts were made to change the county's name, but both failed. Federal census records show it as Davis County from 1860 through 1880, in addition to the 1885 Kansas State census. In 1888 the county was renamed in honor of John W. Geary, an early Governor of the Kansas Territory.