In 1944 (with Giorgio Alberti), Boano bought the Carrozzeria Ghia in Torino when their friend Giacinto Ghia died. Boano and Luigi Segre were central in several low-roofline designs; the Alfa Romeo 2500 CC, Lancia Aurelia (1950), Karmann Ghia (1953), Chrysler K200, Alfa Romeo 1900SS, and some Ferrari 166 of berlinetta style. He is also credited with Lancia Aurelia GT coupe design.
In 1954 he founded Carrozzeria Boano in Grugliasco with his son Gian Paolo Boano (born 1930) who also had been with him while at Ghia. They took over some of Ferrari 250 GT production from Pininfarina. After only three years, Boano closed, and his son-in-law Ezio Ellena took over the 250 production in his Carrozzeria Ellena (1957–1966).
Starting in 1957, Boano worked under Dante Giacosa for Fiat in their Turin styling department, creating the Fiat 600 and the square style of Simca 1000. His son Gian Paolo Boano (1930–) succeeded him in 1959, as leader of the Centro Stile, Fiats styling department.
Mario retired from Fiat in 1966. Gian Paolo remained with Fiat until 1988. Mario Boano died in 1989.