Drinks under lab conditions. Under natural conditions, probably seeks water, but also obtains water from food and dew. Moderate to dense shrub canopy, with or without rock piles or rock outcrops, or in sparse understory with oak woodland overstory. Prefers mesic environments with abundant ground cover. Active above ground all year.
Females are territorial against other females during the breeding season.
Brush mice are solitary nesters. Gestation is 23 days for nonlactating females and 26-32 days for lactating females. Females sexually mature on average at 51 days. The brush mouse is an opportunistic granivore. Likely competitors are Peromyscus maniculatus, P. truei, and P. californicus.