Location: Riverside County, California

Named for Charles Thomas (ca. 1825-95), a pioneer settler in this area. After arriving in California (1849), he acquired and then sold interest in mines at Temescal and received 200 cows in partial payment. He traded 22 of them for 4000 acres of land to start his ranch (1861), where he raised cattle and later race horses. This area was known as Hemet Valley to 1861, then Thomas Valley to 1904, when Thomas sold his now 7000 acre property to stockman Robert Garner, who in turn renamed it Garner Valley. Ernie Maxwell, a local history expert, relates that Thomas named the peak for his wife Geneviva, "because the valley was already named for him".

The mountain was known to the local Indians as Te'llena (Cahuilla), or To'okwe (Serrano) and it was an important ceremonial site. There is a tradition that the valley was once the site of an Indian village (name unknown). A trail led from it along the north slope to the summit to this mountain. However, names were recorded elsewhere: a large village on the south slope was called Palpiswa and a major village to the southeast was called Pasiawha.

The USFS constructed a fire lookout here consisting of a 60' open x-brace steel tower with a 7' by 7' metal cab (1938). Nearby a Ranger outpost and an AWS ground observer station once existed (1936-45). All except the foundation slabs are now gone.

Name is derived from the original Thomas Mountain, cited by the USGS (1897-98), as taking in most of Horse Creek Ridge-now called Little Thomas Mountain.

Name first appears on USGS San Jacinto quad (1901).

Peak was on the original 1946 HPS Peak List.