He was born in Cerignola, in the province of Foggia, in 1882, to Giuseppe Pitassi Mannella and Amalia Conti, the third of seventeen children.

Also known as Errico, after primary school he attended classical studies in a Jesuit college in Mondragone, in the province of Caserta.

He participated in the First World War with the 1st Heavy Field Artillery Regiment, with the rank of captain and later major, being awarded three bronze medals of military valor for actions on Monte Cengio in May and September 1916 and on the heights near Monfalcone on 12 May 1917.

In the same year he accompanied Colonel Alessandro Pirzio Biroli on a mission at the military academy of Ecuador and held a series of lectures on technical subjects of artillery in various foreign countries, especially in Spain.

On 29 June 1940 Mannella was tasked with the investigation into the death of Italo Balbo, governor of Libya, shot down over Tobruk on the evening of the previous day in a friendly fire incident.