Thomas Robert Malthus travelled through Norway in 1799 and his diaries from the trip includes a description of Mjøsa. Malthus wrote that Mjøsa appears as both lake and river because the shores are defined by mountains and where the valley becomes wider the water fills the space. Below Minde (Minnesund) the lake only appears like a river and is called Vorma on the map, according to Malthus.
From its southernmost point at Minnesund in Eidsvoll to its northernmost point in Lillehammer it is 117 km long. At its widest, near Hamar, it is 15 km wide. It is 365 km² in area and its volume is estimated at 56 km³; normally its surface is 123 metres above sea level, and its greatest depth is 468 metres. Its total coastline is estimated at 273 km, of which 30% is built up. Dams built on the distribuary of Vorma in 1858, 1911, 1947, and 1965 raised the level by approximately 3.6 metres in total. In the last 200 years, 20 floods have been registered that added 7 metres to the level of Mjøsa. Several of these floods inundated the city of Hamar.
The cities of Hamar, Gjøvik, and Lillehammer were founded along the shores of the lake. Before the construction of railways past the lake, it was an important transport route. Today, aside from minor leisure boating and the steamship Skibladner, there is no water traffic on the lake. Most of its shores are dominated by rolling agricultural areas, among them some of the most fertile grainlands in Norway. The main train line, the Dovre Line between Oslo and Trondheim, goes along its eastern shore, making stops in Hamar and Lillehammer. From the south European route E6 runs along the eastern shore of the lake until the Mjøsa Bridge connects Moelv on the east with Biri on the west.
The largest and only island is Helgøya. Except for Helgøya, Mjøsa only contains small islets. The most interesting of these is Steinsholmen, which holds the ruins of Mjøskastellet, a medieval citadel dating from the 13th century. Established by King Haakon IV of Norway, it was first mentioned in a letter dated 1234. Peter Andreas Blix documented the site and made drawings in 1897. Hedmark Museum has a future archaeological plan for the site.
Lake Mjøsa has 20 species of fish. Among the most common are pike, European perch, common roach, greyling, and the hundertrout, a brown trout which can reach a weight more than 20 kg. Another common species is the European smelt, which is the most important baitfish for the predators. Historically, the most economically significant species is the lågsild (European cisco).