Niagara is a city in Marinette County, Wisconsin, United States. The population was 1,602 at the 2020 census. The city lies to the north o…Niagara is a city in Marinette County, Wisconsin, United States. The population was 1,602 at the 2020 census. The city lies to the north of and adjacent to the Town of Niagara. It is part of the Marinette, WI–MI Micropolitan Statistical Area.