Mangum is a city in and county seat of Greer County, Oklahoma, United States. The population was 2,762 as of the 2020 United States censu…Mangum is a city in and county seat of Greer County, Oklahoma, United States. The population was 2,762 as of the 2020 United States census. Mangum 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. It became part of the Oklahoma Territory in 1896, and thus part of the state of Oklahoma on November 16, 1907.