Ohio County, the thirty-fifth county in order of formation, is in the Western Coal Field region of Kentucky. It is bordered by Breckinridge, Butler , Daviess, Grayson , Hancock, McLean , and Muhlenberg counties and has an area of 596 square miles. The county was established from part of Hardin County on December 17, 1798, and was named for the Ohio River , which formed its northern boundary until Daviess County (1815) and Hancock County (1829) were created from it. Hartford is the county seat.

The topography of Ohio County is undulating and well suited for agriculture. County farms produce light and dark air-cured tobacco, soybeans, corn, cattle, and hogs. An attempt was made at cultivating silkworms there during 1842-48. The principal waterways are Green River , Rough River, and various creeks.

Large burial mounds found along the Green River in the southern part of the county indicate that the area was once extensively occupied by prehistoric people. Excavations there in the late 1930s by the University of Kentucky and the Works Progress Administration uncovered more than 1,200 skeletons at Indian Knoll. The first pioneers in Ohio County experienced several bloody encounters with the Indians, starting in the 1780s. In 1790 Barnett's Station (now Calhoun) was attacked and two children were killed. Attacks continued into August of the same year. After 1797, the raids ceased to be a danger.

Daniel Boone and Joseph Barnett were among the first surveyors in the region. A Maryland Methodist minister, Ignatius Pigman, was a land speculator credited with bringing in a large number of settlers. One of the first physicians in the county, Dr. Charles McCreery, arrived around 1807.

River traffic down the Green and Rough rivers promoted the county's growth. Hartford became a riverport and mill town on Rough River . On Green River the major towns were Smallhous, Ceralvo, Rockport, and Cromwell. The river traffic was disrupted during the Civil War and dealt a serious blow by the advent of railroads to the county in the 1870s.

During the Civil War, Ohio County was the scene of intense guerrilla activity. On July 21, 1864, a partisan force, commanded by Capt. Dick Yates, ambushed a detachment of Daviess County Home Guards at Rough River Creek, killing four of the Guard. On February 20, 1865, a group of Grayson County Home Guards attacked an encampment of guerrillas near Hartford. Six of the guerrilla force were killed and four wounded. The most damaging event of the war in Ohio County occurred December 20, 1864, when Confederate Gen. Hylan B. Lyon's troops captured the county seat of Hartford and burned the courthouse.

Extensive coal mining took place in Ohio County after the Elizabethtown & Paducah Railroad (now the Paducah & Louisville) came through the county in 1871. 

In 1912 oil was discovered four miles east of Hartford, and since then Ohio County has consistently been one of western Kentucky's leading oil producing counties.

By 1986 the county economy was a mixture of coal mining, agriculture, and oil. The county's incorporated cities in order of size were Beaver Dam , Hartford, Fordsville, McHenry, Rockport, and Centertown. The unincorporated village of Rosine, which was established in 1872 eight miles east of Hartford, is best known as the birthplace of Bill Monroe, the "Father of Bluegrass Music."