There are 4 US Post Offices in Barton County serving a total of 11,952 residents equating to 2,988 residents per post office. It's estimated that approximately 19,077 packages are handled across the 4 county post offices annually. Barton County is a county located in the southwestern part of the U.S. state of Missouri. As of the 2010 census, the population was 12,402. Its county seat is Lamar. The county was organized in 1855 and named after U.S. Senator David Barton from Missouri. President Harry S. Truman was born in Barton County in 1884. The female bandit, Little Britches, the companion in crime of Cattle Annie, was born in Barton County in 1879."