Born in Pieve di Teco in 1897, he entered the Naval Academy of Livorno in 1912, graduating it in 1916 with the rank of ensign after having participated in the Italo-Turkish war as a cadet officer on the training ship Flavio Gioia.

Back in Italy, he became Chief of Staff of the V Naval Division and then commander of the flotilla leader Leone, stationed in East Africa during the Second Italo-Ethiopian War.

Following the Allied landings in Sicily in July 1943 and Patton's advance in western Sicily, his replacement at the command of the Trapani Naval Fortress Area with an Army general was arranged, but Manfredi refused to abandon his men and remained in Trapani, where on 23 July 1943 he fell prisoner of the Americans, after having rendered that port and that of Marsala unusable, and destroying the fuel and ammunition depots to prevent them from falling into enemy hands.

After signing the surrender of the five thousand men under his command, Manfredi handed his saber to General Matthew Ridgway, flatly refusing to reveal the location of the mines scattered in the port to render it unusable.

Manfredi was a prisoner in the United States until February 1944, when he was repatriated following the new co-belligerent status of Italy; in 1945 he was denounced by a Trapani city committee for the destruction of the port, but was acquitted during the preliminary investigation, having only fulfilled his duties as a soldier.