Mangum is a city in and county seat of Greer County, Oklahoma, United States. The population was 3,010 at the 2010 census. It was originally part of Old Greer County in the Texas panhandle. The community was named for A. S. Mangum, who owned the land on which the town was founded in 1882.[a] It became part of Oklahoma Territory in 1896, and thus part of the state of Oklahoma on November 16, 1907.