Monroe is a city in Benton County, Oregon, United States. The population was 763 as of the 2023 census population estimates. It is part of the …Monroe is a city in Benton County, Oregon, United States. The population was 763 as of the 2023 census population estimates. It is part of the Corvallis, Oregon Metropolitan Statistical Area. Monroe is located midway between Eugene and Corvallis along Highway 99W.