It has a surface area of 1,013 km², an elevation of 250 metres above mean sea level, and a shoreline length of 525 km. Viewed from above, the lake consists of a series of finger-shaped flooded valleys, of which 554 km² are in Chile and 459 km² 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 near O'Higgins Glacier, and its characteristic milky light-blue color comes from rock flour suspended in its waters. It is mainly fed by the Mayer River and other streams, and its outlet, the Pascua River, discharges water from the lake towards the Pacific Ocean at a rate of 510 m³/s. The O'Higgins Glacier flows eastwards towards the lake, as does the Chico Glacier. Both of these glaciers are part of the Southern Patagonian Ice Field which extends for approximately 350 kilometres in a north-south direction to the west of Lake O'Higgins.
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 most common tourist route for visiting the lake is that between El Chaltén in Argentina and Villa O'Higgins in Chile, including a ferry through the lake on the Chilean side.
Being the most irregular of the lakes in the area, consisting of eight well defined arms, the name San Martín is sometimes used to refer only to the Argentine side, and O'Higgins to the 4 Chilean arms. Both names come from independence heroes José de San Martín of Argentina and Bernardo O'Higgins of Chile, who fought together for the liberation of Chile, and came to be known as Liberators of America together with other South American figures.
The 4 Argentine arms of the lake, with an area of 521 km², are named Cancha Rayada, Chacabuco, Maipú and De la Lancha, after battles of General San Martín.