Hazen is a city in Mercer County, North Dakota, United States. The population was 2,281 at the 2020 census. Hazen was founded in 191…Hazen is a city in Mercer County, North Dakota, United States. The population was 2,281 at the 2020 census. Hazen was founded in 1913. Hazen has a K–12 school system. It is located about fifteen minutes south of Lake Sakakawea, the largest lake in North Dakota and the third largest man-made lake in the United States.