FM 518 was originally designated in 1945 on a route from U.S. Route 59 (US 59) (now US 90 Alt.) south of Houston to the Brazoria County line. It was extended to Kemah later that year, but a section from US 75 (now Interstate 45) to the Brazoria County line was cancelled in 1946. A 9.5-mile-long (15.3 km) section from the Brazoria County line to US&nsbp;75 was added in 1949. In 1968, the eastern terminus was relocated to SH 146 and a 1.3-mile-long (2.1 km) section of FM 1266 was added to FM 518. The old route was renumbered as FM 2094. An 8.3-mile-long (13.4 km) section from US 90 Alt. to FM 518 junction was transferred to FM 865 in 1984 which brought FM 518 to its current configuration. It was rerouted over FM 3344. In 1995, the entire route of FM 518 was transferred to Urban Road 518 (but is still signed as FM 518).