San Bruno is a city in San Mateo County, California, United States, incorporated in 1914. The population was 43,908 at the 2020 United …San Bruno is a city in San Mateo County, California, United States, incorporated in 1914. The population was 43,908 at the 2020 United States Census. The city is between South San Francisco and Millbrae, adjacent to San Francisco International Airport and Golden Gate National Cemetery; it is approximately 12 miles south of Downtown San Francisco.