Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Sumner County is a county located in the U.S. state of Tennessee. As of 2000, the population was 130,449. Its county seat is Gallatin, but its largest town is Hendersonville. The county is named after Revolutionary War figure Jethro Sumner. Sumner County is part of the Nashville-Davidson Mufreesboro Franklin Metropolitan Statistical Area. Sumner County is located in Middle Tennessee along the northern boundary of the state, on the border with Kentucky. The Cumberland River was important to early trade and transportation, as it merges with the Ohio River to the west.