Viewed from above, the lake consists of a series of finger-shaped flooded valleys, of which 554 square kilometres (214 sq mi) are in Chile and 459 square kilometres (177 sq mi) in Argentina, although sources differ on the precise split, presumably reflecting water level variability.

The lake is the deepest in the Americas with a maximum depth of 836 metres (2,743 ft) near O'Higgins Glacier,[1] and its characteristic milky light-blue color comes from rock flour suspended in its waters.

Both of these glaciers are part of the Southern Patagonian Ice Field which extends for approximately 350 kilometres (220 mi) in a north–south direction to the west of Lake O'Higgins/San Martín.

Immigrants did not settle in the arid windy area around the lake until the 1910s, when British, Scandinavians and Swiss started raising sheep for wool.

The four Argentine arms of the lake, with an area of 521 km2, are individually named Cancha Rayada, Chacabuco, Maipú and De la Lancha, after battles of General San Martín.