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. The rural Greer County is home to Quartz Mountain State Park, near the community of Lone Wolf, Oklahoma. It is also home to the Oklahoma State Reformatory, located in Granite. Its population has declined since 1930 due to changes in agriculture and migration to cities for work.

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.