The area was inhabited for thousands of years by Native Americans including the Tachi Yokuts tribe. They continue to live in the area on the Santa Rosa Rancheria. It was colonized by Spain, Mexico and the United States.

An 1805 expedition probably led by Spanish Army Lieutenant Gabriel Moraga recorded discovering the river, which they named El Rio de los Santos Reyes (River of the Holy Kings) after the Three Wise Men of the Bible. At the time of the United States conquest of California in 1848, the new government changed the name to Kings River after which the county was named.

In 1880, a dispute over land titles between settlers and the Southern Pacific Railroad resulted in a bloody gun battle on a farm 5.6 mi (9.0 km) northwest of Hanford; seven men died. This event became known as the Mussel Slough Tragedy.

Kings County was formed in 1893 from the western part of Tulare County. In 1909, by an act of the state legislature, 208 square miles (540 km2) of Fresno County territory was added to the northwest portion of Kings County.

Settlers reclaimed Tulare Lake and its wetlands for agricultural development. In surface area, it was formerly the largest body of freshwater west of the Great Lakes, and supported a large population of migratory birds as well as local birds and wildlife. Monoculture has sharply reduced habitat for many species.

In 1928, oil was discovered in the Kettleman Hills located in the southwestern part of Kings County. The Kettleman North Dome Oil Field became one of the most productive oil fields in the United States.

During the Great Depression, over 18,000 cotton pickers in the southern San Joaquin Valley, mostly migrant Mexican workers, went on strike in the California agricultural strikes of 1933. During the strike, 3,500 striking farm workers lived in a four-acre camp on the land of a small farmer on the outskirts of Corcoran. Ultimately, the federal government intervened to force both sides to negotiate a settlement.

Lemoore Army Airfield was established for training and defense during World War II. In 1961, the U.S. Navy opened NAS Lemoore 9 miles (14 km) west of Lemoore, not far from the earlier site.

The completion of the California Aqueduct in the early 1970s brought needed water for agriculture and domestic use to the west side of the county.