The city lies on hills and coastal dunes, immediately north of the mouth of the Maipo River and is crossed in two sections by estuaries, of Arévalo to the north and of El Sauce in the section of Llolleo.
The city is located at 33°35′S and 71°37′W. It is bordered on the north by the commune of Cartagena; on the east, by the commune of Melipilla and the Maipo River; on the south, by the same river and by the communes of Santo Domingo and San Pedro; and on the west, by the Pacific Ocean (here termed "el mar Chileno", "the Chilean sea").
It comprises an area of 502.5 km2 (194 sq mi).
San Antonio enjoys a Mediterranean climate with coastal influences. The average annual temperature is 14 °C (57 °F) with averages of 19 °C (66 °F) in January and 8.2 °C (46.8 °F) in July. Annual precipitation of 570 mm (22 in) is concentrated in June, with an average of 134 mm (5 in).
Nomadic hunters and gatherers were the first inhabitants of Chilean territory. In this central coastal zone are evidences of ancient habitation extending back about thirteen thousand years.
The city was 80% destroyed by the 1985 Santiago earthquake.
The San Antonio port was shut down by the 27 February 2010 earthquake, but had resumed operation at 80% capacity by 3 March 2010. After the quake, only 5 of the 8 docking points at the port resumed operation.
According to the 2002 census of the National Statistics Institute, San Antonio spans an area of 404.5 km2 (156 sq mi) and has 87,205 inhabitants (42,843 men and 44,362 women). Of these, 83,435 (95.7%) lived in urban areas and 3,770 (4.3%) in rural areas. The population grew by 11.6% (9,047 persons) between the 1992 and 2002 censuses.
As a commune, San Antonio is a third-level administrative division of Chile administered by a communal council, headed by an alcalde who is directly elected every four years. The 2008-2012 alcalde is Omar Vera Castro, who is advised by six councilors:
Within the electoral divisions of Chile, San Antonio is represented in the Chamber of Deputies by Mrs. María José Hoffmann (UDI) and Mr. Víctor Torres (PDC) as part of the 15th electoral district, (together with Santo Domingo, Cartagena, El Tabo, El Quisco, Algarrobo and Casablanca). The commune is represented in the Senate by Francisco Chahuán Chahuán (RN) and Ricardo Lagos Weber (PPD) as part of the 6th senatorial constituency (Valparaíso-Coast).