It is endemic to the Transverse Ranges of southern California, where it is known only from the San Gabriel and San Bernardino Mountains.

It grows in rocky and disturbed habitat, such as roadsides and scree.

Diplacus johnstonii is an annual herb producing a thin, hairy stem up to about 20 centimeters tall.

The oppositely arranged pointed oval leaves are 1 to 3 centimeters in length.

The tubular base of each flower is encapsulated in a reddish, hairy calyx of sepals with spreading, pointed lobes.