Located on the east side of the park, the Going-to-the-Sun Road parallels the lake along its north shore. At an altitude of 4,484 feet (1,367 m), Saint Mary Lake's waters are colder and lie almost 1,500 feet (460 m) higher in altitude than Lake McDonald, the largest lake in the park, which is located on the west side of the Continental Divide. Here, the great plains end and the Rocky Mountains begin in an abrupt 5,000 feet (1,500 m) altitude change, with Little Chief Mountain posing a formidable southern flank above the west end of the lake. St. Mary lake has a surface area of 3,923 acres (15.88 km2).
The lake is 9.9 miles (15.9 km) long and 300 feet (91 m) deep. The waters of the lake rarely rise above 50 °F (10 °C) and are home to various species of trout. During the winter, the lake often is frozen completely over with ice up to 4 feet (1.2 m) thick.
The famous opening scene from the 1980 Stanley Kubrick movie The Shining was shot at Saint Mary Lake.