Gilmer is a town in and the county seat of Upshur County, Texas, United States. It is best known for being the home of the East Texas Yamboree, as well as the birthplace of popular music singers Don Henley of the Eagles band and Johnny Mathis, as well as blues musician Freddie King. The population was 4,905 at the 2010 census. The city's namesake is a former United States Secretary of the Navy, Thomas Walker Gilmer.