The lake is 27 miles (43 km) long with 73 percent of the lake's surface area in Quebec, where it drains into the Magog River. However, three-quarters of its watershed, 489 square miles (1,270 km2), is in Vermont. The total is 687 square miles (1,780 km2), with 198 square miles (510 km2) located in Quebec. In Vermont, the lake lies in parts of the towns of Derby and Newport, in addition to the city of Newport, all in Orleans County. In Quebec, the lake lies in parts of Austin, Magog, Ogden, Potton, Saint-Benoît-du-Lac, and Stanstead Township, all in Memphrémagog Regional County Municipality.
The lake elevation is 682 feet (208 m) and its maximum recorded depth is 285 feet (87 m) located near the international divide. The lake is the third deepest in Vermont It contains 20 islands. Province Island, the largest, is divided by the international border.
The name Memphremagog is derived from Algonkian, in which it means "where there is a big expanse of water".
The lake is site of La Traversée Internationale du Lac Memphrémagog, a professional and amateur swimming extravaganza in Magog, Quebec, Canada, and In Search of Memphre, an international amateur marathon swim between Newport, Vermont, U.S.A. and Magog, Quebec, Canada.