After the fall of the Fascist regime on 25 July 1943, the new head of the government, Marshal Pietro Badoglio, entrusted Montezemolo with the direction of his secretariat; he was also appointed commander of the 11th Armored Engineers Group.

Following the announcement of the Armistice of Cassibile, while the king and the government fled from Rome, Montezemolo remained in the capital; he was part of the Italian delegation that negotiated with Field Marshal Albert Kesselring the conditions of the ceasefire in the capital on 10 September 1943, after which General Giorgio Calvi di Bergolo, appointed commander of the "Open City" of Rome, made him head of the Civil Affairs Office of the Open City Command.

Outside of cities, several partisan groups of Italian soldiers in uniform and former prisoners of war were organized and coordinated by Montezemolo throughout central Italy, such as the Raggruppamento Monte Amiata.

[1][2][6][7][8][9] Montezemolo worked hard to coordinate with the other elements of the Roman National Liberation Committee and in particular with Communist Giorgio Amendola, with whom he planned military operations following the Anzio landing.

[1][2] On 25 January 1944, at the end of a clandestine meeting with General Quirino Armellini, Montezemolo was arrested by the Nazis together with his friend Filippo De Grenet, also a member of the Resistance.