Thomas Mountain is a summit in Riverside County's Peninsular Ranges that marks the northern edge of the Anza Valley. The community of Thomas Mountain lies on its east side, while Bautista Canyon separates it from nearby Cahuilla Mountain. Beyond its geographic prominence, the mountain served in the region's fire-watch system. In 1935, the Thomas Mountain Lookout and its residence were built there, and during World War II the site also functioned as a plane-spotting post. After suffering damage from fire and vandalism, the lookout was demolished and scrapped in 1973.