[3] Its name is reflective of its origins, being the union of portions of Franklin, Delaware, Madison, and Logan counties.

Union County has been under the rule of three countries in its history: France, England, and the present-day United States.

[7] A notable victim of the warring between the settlers and Indians was Jonathan Alder, who resided in and around the Union County area throughout his life.

An important settlement made in the county was by Abraham Amrine, of Swiss descent and a revolutionary soldier, in 1817, two miles (3.2 km) northwest of Marysville.

After lobbying by Col. James Curry, who represented the area in legislature at the time and a resident of what would become Jerome township, Union County was established by the Ohio legislature, and became official in 1820 with the appointment of the original commissioners: Stephen Bell, Reuben Wallace, and John Huston.

The county commissioners eventually established the townships, including Union (1820), Darby (1820), Mill Creek (1820), Jerome (1821), Paris (1821), Liberty (1822), Leesburg (1825), Allen (1827), Jackson (1829), York (1834), Washington (1836), Dover (1839), Claibourne (1834), and Taylor (1849), while they designated Marysville as the county seat.

The first county jail was a log structure that sat on the southside of East Center street, in the rear of the courthouse.

[15] In 1866, the Union County Teachers' Institute was organized in Marysville, with Franklin Wood serving at the first President.

In 1844, the legislature by act created a free turnpike that ran from Columbus to Bellefontaine, passing through Marysville, originally costing $230,000.

A railroad called the New York, Pennsylvania, and Ohio line was chartered in 1851 and would eventually run through Union County with station points at Richwood, Woodland, Claibourne, Pottersburg, Broadway, and Peoria.

The Toledo and Central railroad ran from Columbus north through the county, with station points at Marysville, Peoria, and Raymond.

The crops grown in Union County's early history included wheat, oats, corn, Irish potatoes, clover and alfalfa.

[20] Union County was home to many industries in its original days from hotels, tanneries, distilleries, breweries, mills, asheries, manufacturers, energy production, banks, grocers, and retailers, among others.

[21] A wool company, Woodbury & Welsh, constructed a brick factory on the northeast part of town in 1864, and a brewery was built in 1866 on the eastside.

There were large tile manufacturers, and a plethora of commercial and industrial interests including jewelers, furniture retailers, and lumber yards.

Edward Stillings grew up near Milford Center and was a graduate of Harvard, and became one of the top jurists in the country, practicing before the United States Supreme Court and helping to form the legal code of Kansas in the 1870s.

Cole, attended Harvard, practiced law with his father in 1867, and after being elected to the legislature in Indiana, became Deputy United States Assessor for Internal Revenue.

Cole and W.C. Lawrence, who renamed it the Argus and Union County Advertiser, and moved the politics toward the Whig party.

Trade was done by barter, including the exchange of coon and deer skins, whiskey, honey, and other durable goods.

A descendant of a hero, Oliver Hazard Perry, from the War of 1812, named Robert S. Beightler, was a native of Union County.

[37] When the Mexican War broke in 1845 following the annexation of Texas into the United States, Union County provided men, totalling over 30.

This was sponsored through private, public, and corporate grants and donations, including support from the Union County Foundation.

[54] There are numerous local family farms in Union County, as well as multinational agricultural services corporations like Scotts Miracle-Gro Company and Select Sires.

His cattle at the Mayflower Farm have produced world records, while he himself has publicly served in positions appointed by the United States Secretary of Agriculture Bob Bergland and Ohio Governor Richard Celeste.

He has been chairman of the Union County Board of Elections, and is an inductee into the Ohio Agricultural Hall of Fame.

Major corporations operating regional facilities in the county include Honda of America, Goodyear/Veyance Technologies, Parker Hannifin, United Rotary Brush, and Invensys Climate Control.

Other large companies located in the county include Sumitomo Electric Wiring Systems, Ray Lewis and Son, Velocys, MAI Manufacturing, NEX Transport, and Midwest Express, among others.

Between 2000 and 2007, Union County businesses were awarded $9.3 million in United States Department of Defense contracts.

[60] In 2010, Stivers defeated her in a rematch,[61] and after reapportionment and redistricting, the county was placed in the heavily Republican 4th district, currently represented by Jim Jordan.

[63] The entire county is contained in the 86th Ohio House district, currently represented by Tracy Richardson elected in 2018.