In June 1916 he captured Cima Caldiera from the Austro-Hungarians on the Asiago plateau, for which he was awarded a Silver Medal of Military Valor, and a year later he participated in the battle of the Ortigara.

[1][3][5][4] On 8 April 1939 he was flown to Tirana at the head of an ad-hoc, regiment-sized unit of the Grenadiers of Sardinia, named after him, in order to participate in the conquest of Albania, for which he was awarded another bronze medal for Military Valor.

Immediately afterwards he became deputy commander of the 21st Infantry Division "Granatieri di Sardegna" (General Taddeo Orlando), sent to Slovenia (where it was stationed between Ljubljana and Kočevje) to garrison the province of Ljubljana and counter partisan activity in the area), but his stay was cut short as from 11 August 1941, on request of general Gastone Gambara (who had been his superior in Greece), he was assigned to the Armed Forces High Command in North Africa, for special assignments.

[6] In early 1943 he raised a division-sized "Saharan group" named after him (Raggruppamento Sahariano "Mannerini"), composed of the remnants of units that had been destroyed during the retreat from Libya (some 6,000 men, including the seven surviving Saharan companies, GAF troops, PAI personnel, colonial troops and cavalry units), which fought under his command during the Tunisian campaign, in the battle of the Mareth Line and in further clashes with the Long Range Desert Group.

He was captured near Gabes on 29 March 1943 and remained in Allied captivity until 1945; upon returning to Italy, on 14 June 1945, he was awarded the Officer's Cross of the Military Order of Savoy.