It has been so named since the 16th century, but was previously called the Lago di San Giulio, after Saint Julius (4th century), the patron saint of the region; Cusio is a merely poetical name. Its southern end is about 35 km by rail NW of Novara on the main Turin-Milan line, while its north end is about 6 km by rail south of the Gravellona-Toce railway station, half-way between Ornavasso and Omegna.
The chief place is Orta San Giulio, built on a peninsula projecting from the east shore of the lake, while Omegna is at its northern extremity. It is supposed that the lake is the remnant of a much larger sheet of water by which originally the waters of the Toce flowed south towards Novara. As the glaciers retreated the waters flowing from them sank, and were gradually diverted into Lake Maggiore.