- Monroe City is a city located in Monroe County, Missouri, United States. It is the county seat of Monroe County. According to the United States Census Bureau, the city has a total area of 1.27 square miles (3.29 km2), all land. Monroe City is known for its historic downtown area, annual events like the Fourth of July celebration, and proximity to Mark Twain Lake, which offers recreational activities such as boating, fishing, and camping. The city has a population of approximately 2,500 people, as of the 2010 census.