Born in Rome, Italy, Magni started his career as a screenwriter, in 1956, with Tempo di villeggiatura.

[1] In 1968 he collaborated with Mario Monicelli in creating a real "event" of the Italian cinema by transforming Monica Vitti into a comedic actress with The Girl with the Pistol, and the critical and commercial success of the film pushed him into directing.

[1] After the directorial debut with Faustina (which was also the debut film of Vonetta McGee), in 1969 Magni achieved an extraordinary success with Nell'anno del Signore, which was the highest-grossing Italian film of the year, so as to require for the first time in Italy nighttime screenings to meet the demands of the audience.

The film also defined Magni's style, namely a commedia all'italiana mainly centred on Rome and its history, particularly the epoch between the Papal States and the Risorgimento.

[2] In 1977 Magni achieved critical recognition with In nome del papa re, which also gave him his first David di Donatello Award.