McMullen County is a county located in the U.S. state of Texas. As of the 2020 census, its population was 600, making it the fourth-least populous county in Texas. Its county seat is Tilden. The county was established from parts of Bexar County, Atascosa County, and Live Oak County in 1858 and later organized in 1877. It is named for John McMullen, founder of a colony in Texas. The McMullen County Courthouse was designed by the architect W.C. Stephenson, originally from Buffalo, New York.