Jefferson County is a county located in the U.S. state of Kentucky. It is the most populous county in Kentucky and is more than twice as populous as the second most populous, Fayette. It was formed in 1780. As of 2008, the population estimate recorded by the U.S. Census Bureau is 713,460.
Jefferson County is the most populous county in the Louisville/Jefferson County, KY–IN Metropolitan Statistical Area.
Jefferson County was organized in 1780 and one of the first three counties formed out of the original Kentucky County, which was still part of Virginia at the time (the other two being Fayette and Lincoln). The county is named for Thomas Jefferson, who was governor of Virginia at the time.
The last major American Indian raid in present day Jefferson County was the Chenoweth Massacre on July 17, 1789.
In 2003, its government merged with that of its largest city and county seat, Louisville, forming a new entity, the Louisville-Jefferson County Metro Government (the official long form) or simply Louisville Metro (the official short form).
Prior to this merger, the head of local government was the County Judge/Executive, a post that still exists but now has few powers. The current incumbent is Ken Herndon. Local government is effectively now led by the Mayor of Louisville Metro, Jerry Abramson.