There are 24 US Post Offices in Marion County serving a total of 632,930 residents equating to 26,372 residents per post office. It's estimated that approximately 1,010,227 packages are handled across the 24 county post offices annually. Marion County is a county located in the U.S. state of Indiana. Census 2010 recorded a population of 903,393, making it the largest county in the state and 55th most populated county in the country, greater than the population of six states. The county seat is Indianapolis, the state capital and largest city. Marion County is consolidated with Indianapolis through an arrangement known as Unigov. Marion County is included in the Indianapolis-Carmel-Anderson, IN Metropolitan Statistical Area."