It was one of two original counties created in the Northwest Territory in 1790 and was gradually reduced in size as subsequent counties were established. It was established in its present configuration when Daviess County was partitioned off in February of 1818. It was named for Major General Henry Knox, who had completed his term as second U.S. Secretary of War the previous September. The county has a total area of 524 square miles to go with a population count of almost 37,000 residents.