Head and body length of Peromyscus species range from 70-170 mm and tail lengths are 40-205 mm. There are 2 subgenera and 55 species. P. californicus has soft dense fur and large ears which are covered with fine hairs. The tail is about one third the length of the total length of this mouse, and is also covered with fine hairs. Females have four or six mammae.

The preferred habitat is deep woodlands and brushy areas. Several nest are built each year and are line with soft down from plants or other materials which are shredded. Once a nest is soiled it is abandoned and another is constructed. P. californicusis primarily nocturnal and active year round, though periods of torpor occur for several hours, during which time there is a drop in body temperature.

The diet consists of a wide variety of foods including nuts, berries, fruits, insects, other small invertebrates, and carrion.

Studies have shown that a paired male and female P. californicus will live together cooperatively raising offspring. males are highly aggressive toward one another. Estrous cycles last an average of 7 days, gestation periods range from 21-27 days and averaging 23.4 days in non-lactating females. Lactating females can have gestation periods of up to 40 days.. Litter sizes vary by region, with those in the northern areas being larger. Overall litter sizes for Peromyscus genus is 3.4 offspring weighing 2.2 grams. Most young are weaned at 3-4 weeks and stay with the mother for another month.

P. californicus occurs from C and S California, USA, excluding the San Joaquin Valley, to NW Baja California Norte, Mexico.