There are 3 US Post Offices in Brown County serving a total of 3,220 residents equating to 1,073 residents per post office. It's estimated that approximately 5,139 packages are handled across the 3 county post offices annually. Brown County is a county in the U.S. state of Nebraska. As of the 2010 census, the population was 3,145. Its county seat is Ainsworth. The county was established in 1883 and named for two members of the legislature who reported the bill for the county's organization. In the Nebraska license plate system, Brown County is represented by the prefix 75 (it had the 75th-largest number of vehicles registered in the county when the license plate system was established in 1922)."