Jefferson county was formed in 1780. It is located in the Outer Bluegrass region of the state. The elevation in the county ranges from 383 to 902 feet above sea level. In 2010 the county population was 741,096 in a land area of 380.42 square miles, an average of 1948.1 people per square mile. It is the most populous county and the most densely populated county in the state.
The county seat is Louisville.
Jefferson was one of the original counties in the Kentucky territory of Virginia. The Louisville and Jefferson county governments are merged into the Louisville/Jefferson County Metro Government.