San Geronimo is bordered by Lagunitas-Forest Knolls to its west and Woodacre to its east.

It is 8 miles (13 km) southwest of downtown Novato[5] at an elevation of 292 feet (89 m).

[4] The first known map to depict the San Geronimo Valley was published in 1834.

[5] The San Geronimo post office opened in 1895, closed in 1910, and re-opened in 1911.

[3] According to the United States Census Bureau, the CDP has a total area of 1.5 square miles (3.9 km2), all of it land.

[7] In 2018, The Trust for Public Land purchased the 157-acre (64 ha) site and has opened it to public access as a park while it works to restore habitat and conduct a planning process for future use.

There were 208 housing units at an average density of 137.9 per square mile (53.2/km2), of which 73.4% were owner-occupied and 26.6% were occupied by renters.

There were 176 housing units at an average density of 116.0 per square mile (44.8/km2).

The racial makeup of the CDP in 2010 was 90.6% non-Hispanic White, 0.7% non-Hispanic African American, 0.4% Native American, 0.7% Asian, 0.4% from other races, and 2.5% from two or more races.