Crosby is a city in and the county seat of Divide County, North Dakota, United States. The population was 1,065 at the 2020 census. Crosb…Crosby is a city in and the county seat of Divide County, North Dakota, United States. The population was 1,065 at the 2020 census. Crosby was founded in 1904 at the end of a Great Northern Railway branch line that began in Berthold. The name comes from S.A. Crosby, a businessman from Portal involved in establishing the city.