In his obituary of Mastroianni, critic Tony Sloman described him as "arguably, the finest Italian film editor of his generation.

He had a significant collaboration with director Federico Fellini, whose films he edited for over twenty years; their work includes Giulietta degli spiriti (1965), Amarcord (1973), and Ginger and Fred (1986), the last of which features his brother.

[1] He had a similarly notable collaboration with director Luchino Visconti in films like Le Notti Bianche (1957), Morte a Venezia (1971), Ludwig (1972) and Gruppo di Famiglia in un Interno (1974).

He also edited the 1974 absurdist western comedy Don't Touch The White Woman!.

He won 5 David di Donatello Awards and 1 Nastro d'Argento as Best Editor.