At the outbreak of the First World War, with the rank of captain, he was assigned to the fort of Punta Corbin in the Val d'Astico (Vicentine Alps); he was later transferred to the Fourth Army.

[1] On 1 February 1940 Pentimalli was given command of the 32nd Infantry Division Marche, stationed near Treviso, and on 31 May, ten days before Italy's entry into World War II, he was promoted to major general.

[1][2][3][4][5] On 26 June 1941 he returned to Italy, and on 13 August he assumed command of the newly formed 151st Infantry Division Perugia, initially stationed in the eponymous city.

He took no action to prevent the German takeover in the area under the control of his troops, leaving his subordinates without orders, and eventually fled in civilian clothes along with General Ettore Deltetto and his chief of staff, only reappearing on 1 October, when the Allies entered Naples.

They were acquitted of the accusation of collaborationism, but sentenced to twenty years' imprisonment for "abandonment of command" on 24 December 1944; the High Court of Justice, while admitting "the overwhelming superiority of the German forces", stated that defenders could have done "something more and better".