San Joaquin is a city in Fresno County, California, United States. The population was 4,001 at the 2010 census, up from 3,270 at the 2000 …San Joaquin is a city in Fresno County, California, United States. The population was 4,001 at the 2010 census, up from 3,270 at the 2000 census. The nearest high school in the area is Tranquillity High School in Tranquillity. San Joaquin is located 11 miles southwest of Kerman, at an elevation of 174 feet.