At age 22, Adalberto Garelli received a degree in engineering and dedicated his work to developing and perfecting the 2-stroke engine for Fiat. Garelli quit in 1911 due to Fiat's lack of enthusiasm for the 2-stroke engine. He continued his own engine design between 1911 and 1914 which resulted in the 350cc split-single. Garelli worked for other motorcycle manufacturers from 1914 to 1918 during which time he won a competition organized by the Italian Army to design a motorcycle with which he used his 350cc split-single engine.
In 1919, Garelli constructed a 350cc motorcycle which set a long distance record from Milan to Naples. Rider Ettore Girardi covered the 840 kilometers with an average of 38.29 km/h. Many famous Italian racers such as Ernesto Gnesa, Tazio Nuvolari and Achille Varzi began their racing careers on Garelli bikes. The Garelli 350cc split-single stayed in production until 1926 and made a major impact in racing. The company also produced motorcycles for the Italian military. After World War II, Garelli concentrated on producing smaller bikes and mopeds for the European market.
In the early 1980s, Garelli dominated the 125 class in Grand Prix motorcycle racing winning six consecutive world championships between 1982 and 1987.