On February 12, 1820, the Ohio government authorized the creation of Henry County, although the county remained a part of Wood County until 1824 and a portion of Williams County until 1834. Residents named the county in honor of Patrick Henry, a hero of the American Revolution. Previously, the county had been part of land reserved to Ohio's Indian people, under the Treaty of Greenville.
Henry County is located in northwestern Ohio in what used to be the Great Black Swamp. It is predominantly rural, with less than one percent of the county's 417 square miles consisting of urban areas. The county seat is Napoleon. With a population of 9,318 people, Napoleon was the county's largest community in 2000. Many residents of Ohio's rural communities are seeking better lives and more opportunities in the state's cities, but Henry County's population seems to be remaining stable. The county experienced a less than one percent decline in population between 1990 and 2000, leaving the total number of residents at 29,210. The county averages seventy people per square mile.
Farming is the largest employer in Henry County, with ninety-one percent of the county as farm fields. Henry County farmers are Ohio's third largest producers of wheat. Manufacturing businesses and retail positions are the second and third largest employers in the county, following well behind farming in the number of people employed. A Campbell Soup plant is the primary manufacturing establishment in the county. In 1999, the per capita income in the county was almost nineteen thousand dollars, with 6.4 percent of the people living in poverty.
Most voters in Henry County claim to be independents, yet in recent years, they have overwhelmingly supported Republican Party candidates at the national level.
Among the county's most notorious residents was backwoodsmen Simon Girty.