There are 13 US Post Offices in Washtenaw County serving a total of 334,847 residents equating to 25,757 residents per post office. It's estimated that approximately 534,453 packages are handled across the 13 county post offices annually. Washtenaw County /\u02c8w\u0252\u0283t\u1d7bn\u0254\u02d0/ is a county located in the U.S. state of Michigan. As of the 2010 census, the population was 344,791. The county seat is Ann Arbor. The county was founded in 1822 and organized as a county in 1826. Washtenaw County comprises the Ann Arbor, MI Metropolitan Statistical Area and is included in the Detroit-Warren-Ann Arbor, MI Combined Statistical Area. The county is home to the University of Michigan, Eastern Michigan University, Washtenaw Community College, Concordia University Ann Arbor, and the Ann Arbor campus of the Thomas M. Cooley Law School."