Mercer County is a county located in the U.S. state of Kentucky. As of the 2010 census, the population was 21,331. Its county seat is Harrodsburg. The county was formed from Lincoln County in 1785 and is named for Revolutionary War General Hugh Mercer, who was killed at the Battle of Princeton in 1777.It is a prohibition or dry county, though Harrodsburg and Pleasant Hill allow the sale of alcohol by the drink.