[1] After graduating as an ensign in 1908, he served for a few years on battleships and cruisers, and in 1911-1912 he participated in the Italo-Turkish War as a sub-lieutenant aboard the hydrographic survey ship Staffetta, in Red Sea.

[1] After the war, Legnani, who had become a lieutenant commander, interchange shore assignments in Aegean, first in the Castelrosso naval base and later in Lakki (Leros), and periods on submarines and destroyers.

[1] In 1931 Legnani was assigned to the Office of chief of staff at the Ministry of the Navy, and in 1933 he was promoted to captain and given command of the light cruiser Alberico da Barbiano.

[1] After another period at the Staff in the Ministry of the Navy, in 1937 Legnani was promoted to rear admiral and placed in command of the Regia Marina's submarine fleet, a position he held for two years; he also directed the clandestine submarine warfare during the Spanish Civil War, for which he was decorated with the Knight's Cross of the Military Order of Savoy.

[1][4] Less than a month later, however, on 20 October 1943, Admiral Legnani lost his life in a car accident near Vicenza,[1][5] His son Emilio also served in the Italian Navy, receiving the Gold Medal of Military Valor for his actions in the Black Sea during World War II.