Mountrail County is a county located in the U.S. state of North Dakota. As of the 2010 census, the population was 7,673. Its county seat is Stanley. Mountrail County is one of several western North Dakota counties with significant exposure to the Bakken Formation in the Williston Basin. In the county the population was spread out with 28.10% under the age of 18, 6.80% from 18 to 24, 23.20% from 25 to 44, 24.20% from 45 to 64, and 17.70% who were 65 years of age or older.