a city in southern California, USA. Originally an area with mud flats, in 1882 San Pedro became a port and in 1909 it was annexed to Los Angeles. It was at this timework began to transform the area by dredging, resulting in the creation of one of the world's largest man-made harbors. The city's waterfront is occupied with shipyards, fish canneries and oil refineries. In addition to the harbor, San Pedro is also the site of a United States military and naval base.a city in southern California that is an important porthandling much of the maritime commerce of Los Angeles and has shipyards, fish canneries and oil refineries.