In November 1941, Nanni left for military service, enlisting in the 6th Engineers Regiment. He spent several months in Bologna where he attended evening classes at the Academy (art school). In 1942, he qualified as a telegraph operator and joined numerous exercises, from Bolzano to Santa Maria Capua Vetere. In April 1943, he was transferred to Rome. On 8 September, Nanni was stationed at the Forte Trionfale Barracks in Monte Mario. In the absence of orders and facing the risk of arrest by German forces, he decided to return home.

A few quiet weeks passed, but on 18 November 1943 he was called up by the Italian Social Republic’s army. Like many fellow villagers, Mario delayed reporting for duty and went into hiding. In early July 1944, he went to Belpoggio to join partisans of the Stella Rossa Brigade and was assigned to the third battalion commanded by Otello Musolesi.

On 28 September 1944, German forces began a round-up between the Setta and the Reno rivers, culminating in the Marzabotto massacre. Mario was in Caprara di Sotto: overwhelmed by attack, the partisans attempted to fall back towards Grizzana, but they were scattered. He followed Guido Fanti eastwards, to reach the Allies. With his friends Ferruccio and Ivo Teglia, he wandered through the mountains, amidst fog and danger, surviving on chestnuts and water. After several difficult days he reached Bellaria, where American forces rescued them. Finally, he reached Serracava and was reunited with his family.

Hired as a municipal clerk, he contributed to the organisation of a temporary town hall whilst awaiting the liberation of Vado, which took place on 19 April 1945. At the end of 1946, Nanni decided to pursue an artistic career in Bologna. After graduating the Academy, he began frequenting artistic circles and joined the Italian Communist Party (PCI).

Following a figurative period, he was among the first Italians to embrace the Informal art movement. In 1955, he held his first solo exhibition and in the following years exhibited successfully in Milan, Rome and Alexandria (Egypt). In 1960, he became a teacher at the Art School and later at the Academy, where he remained until his retirement. In 1984 he attended the Venice Biennale.

In 1996 Nanni recounted his experience at Montesole and in 1997 created the War Memorial in Monzuno.

Significantly, Nanni’s last exhibition was dedicated to the Resistance. He passed away on 23 November 2019 in Bologna.