Roger Mills County is a county located in the western part of the U.S. state of Oklahoma. As of the 2010 census, the population was 3,647, making it the third-least populous county in Oklahoma. Its county seat is Cheyenne. The county was created in 1891.
The county is located above the petroleum-rich Anadarko Basin.
Roger Mills County takes its name from Roger Q. Mills, a senator from Texas. The town of Cheyenne in Roger Mills County is the location of the Battle of Washita River (also called Battle of the Washita; Washita Battlefield and the Washita Massacre), where George Armstrong Custer’s 7th U.S. Cavalry attacked Chief Black Kettle’s Cheyenne village on the Washita River on November 26, 1868.
The area covered by Roger Mills County had been part of the Cheyenne Arapaho reservation until after Oklahoma Territory was created and County E was formed. County E was renamed Day County. Day County was abolished and Roger Mills County was created at statehood on November 16, 1907. The county's western boundary with Texas was moved eastward when the Supreme Court ruled that the 100th Meridian was farther east than originally supposed.
During the 1970s Roger Mills County and the surrounding area would benefit from the natural gas and oil development in the Panhandle-Hugoton field, the largest-volume gas field in the United States, and the world’s largest known source of helium. Between 1973 and 1993 the field produced over 8-trillion cubic feet (230,000,000 m³) of gas.