Mountain House lies on the foothills of the Diablo Range, and close to the Altamont Pass which is over 1000 feet in elevation. It is on the border of Alameda and Contra Costa Counties. According to the United States Census Bureau, the CDP covers an area of 3.2 square miles (8.3 km2), all of it land. The community is bisected by Mountain House Creek. Mountain House Creek originates south of the Altamont Pass and Interstate 580, flowing northeasterly along and crossing under the interstate, then along Grant Line Road to the intersection with Mountain House Road at the historic Alameda County Mountain House. The creek continues northeasterly through the residential communities of Mountain House before emptying into the Old River, a distributary of the San Joaquin River.