Mountrail County is a county in the northwestern part of North Dakota, United States. As of the 2020 census, the population was 9,809. Its county seat is Stanley. The county was originally created in 1873, then removed in 1892, annexed by Ward County. It was re-created and organized in 1909.

Stanley is a city in Mountrail County, North Dakota, United States. It is the county seat of Mountrail County. The population was 2,321 at the 2020 census, making it the 22nd largest city in North Dakota. Stanley was founded in 1902. The town's economy is heavily connected to the nearby oil-rich Bakken Formation.