There are 9 US Post Offices in Buchanan County serving a total of 76,909 residents equating to 8,545 residents per post office. It's estimated that approximately 122,755 packages are handled across the 9 county post offices annually. Buchanan County is a county located in the U.S. state of Missouri. As of the 2010 United States Census, the population was 89,201. Its county seat is St. Joseph. When originally formed in 1838, the county was named Roberts County, after settler Hiram Roberts; it was renamed in 1839 for James Buchanan, then a U.S. Senator and later President of the United States. The county was formed from land annexed to Missouri, as were five other counties. Buchanan County is included in the St. Joseph, MO-KS Metropolitan Statistical Area.