Jefferson is an unincorporated community in Washington Township, Clinton County, Indiana. The town is named for U.S. President Thomas Jefferson. Jefferson was one of the earliest settlements in Clinton County, being laid out and platted before 1829 and situated in the most thickly settled part of the county at that point. It early on had a post office, w…Jefferson is an unincorporated community in Washington Township, Clinton County, Indiana. The town is named for U.S. President Thomas Jefferson. Jefferson was one of the earliest settlements in Clinton County, being laid out and platted before 1829 and situated in the most thickly settled part of the county at that point. It early on had a post office, which handled mail bi-weekly, and a store operated by Abner Baker and Aaron Southard which sold supplies both to the early pioneers and to the Indians.