Knox County is a county located in the U.S. state of Maine. As of the 2010 census, the population was 39,736. Its county seat is Rockland. The county is named for American Revolutionary War general and Secretary of War Henry Knox, who lived in the county from 1795 until his death in 1806. The county was established on April 1, 1860, and is the most recent county to be created in Maine. It was carved from parts of Waldo and Lincoln counties. The Union Fair, started in 1868, began as the efforts of the North Knox Agricultural and Horticultural Society.