Greer County is a county located along the southwest border of the U.S. state of Oklahoma. As of the 2020 census, the population was 5,491. Its county seat is Mangum. From 1860 to 1896, the state of Texas claimed an area known as Greer County, Texas, which included present-day Greer County along with neighboring areas. In 1896 it was designated as a county in Oklahoma Territory under a ruling by the US Supreme Court.