3900 Fm 1979 None is located in San Marcos, Texas, in Guadalupe County. Guadalupe County anchors the fast-growing corridor between San Antonio and Austin, centered on Seguin, one of the oldest Anglo settlements in Texas, founded in 1838 and named for Juan Seguín, the Tejano hero of the Texas Revolution. The county blends deep Texas history with explosive modern growth driven by its position between two of America's largest metro economies.

The Guadalupe River flows through the heart of Guadalupe County, providing river frontage properties and water recreation within easy reach of San Antonio. Lake Gonzales and Meadow Lake provide additional water amenities. The terrain transitions from the limestone hills of the western county to the rich blackland prairie soils of the east, which have supported agriculture since the earliest Anglo settlements.

San Marcos had an estimated population of 68,920 as of 2023. The 78666 zip code has a median household income of $55,478, a median home value of $271,900, and a median age of 27. The average household size is 2.35 people.

Guadalupe County is one of the fastest-growing counties in Texas, driven by San Antonio and Austin metro expansion, major manufacturing including the massive Toyota truck plant in nearby San Antonio, and a surging residential real estate market. Seguin's industrial base includes significant advanced manufacturing alongside the growing bedroom community residential sector serving both metro areas.