Buchanan County is a county located in the U.S. state of Missouri. As of the 2010 U.S. 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.