Greencastle is a city in Greencastle Township, Putnam County, Indiana, United States, and the county seat of Putnam County. It was founded in 1821 by English American Ephraim Dukes on a land grant. He named the settlement for his hometown of Greencastle, Pennsylvania. Greencastle was a village or town operating under authority of the Putnam County commissioners until March 9, 1849, when it became a town by special act of the local legislature. Greencastle, Indiana, officially became a city after an election held on July 8, 1861. The first mayor of Greencastle was E. R. Kercheval, a member (during his lifetime) of the Freemason Temple Lodge #47. The city became the county seat of Putnam County. The population was 10,326 at the 2010 census.