Buchanan County is located in the U.S. state of Missouri. As of the 2020 United States Census, the population was 84,793. 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.

St. Joseph is a city in Andrew and Buchanan counties and the county seat of Buchanan County, Missouri, United States. Located on the Missouri River, it is the principal city of the St. Joseph Metropolitan Statistical Area, which includes Buchanan, Andrew, and DeKalb counties in Missouri and Doniphan County, Kansas. As of the 2020 census, St. Joseph had a total population of 72,473, making it the 8th most populous city in the state, and the 3rd most populous in Northwest Missouri. St. Joseph is located roughly thirty miles north of the Kansas City, Missouri, city limits and approximately 125 miles (201 km) south of Omaha, Nebraska.