Monroe is the ninth-largest city in the U.S. state of Louisiana, and is the parish seat and largest city of Ouachita Parish. With a 2020 census-t…Monroe is the ninth-largest city in the U.S. state of Louisiana, and is the parish seat and largest city of Ouachita Parish. With a 2020 census-tabulated population of 47,702, it is the principal city of the Monroe metropolitan statistical area, the second-largest metropolitan area in North Louisiana.