San Mateo thornmint is an annual herb, living less than one year. Flowers appear from April through June or July, with those lower on the stem typically opening first. Opening three to five hours after sunrise, the flowers remain open for two to four days. San Mateo thornmint is pollinated by generalist pollinators, including native bees. The plants can also reproduce by pollinating themselves, in which case pollinators are not necessary. Each plant can produce a large number of seeds. Seeds are thought to require six months of dormancy after they fall and can survive at least 2.5 years in the soil.

San Mateo thornmint is only found in San Mateo County. It grows on serpentine clay soils that fill fissures between other types of rock and soil. These clays have a low calcium/magnesium ratio, as well as a high moisture content. General community associates include chaparral and valley foothill grassland plants.

San Mateo thornmint is a strongly scented annual herb of the mint family (Lamiaceae). The plant can grow up to 8 inches tall and is typically unbranched. It has a squarish stem and opposite leaves. The oblong leaves are small, between 0.3 to 0.5 inches, and may be toothed. The flowers are white or sometimes tinged with lavender and grow in tight clusters surrounded by very spiny bracts, which are small scale-like leaves that grow beneath the base of the flowers.