Trezzani was born in Savigliano, Piedmont, and joined the Royal Italian Army in his youth, enlisting in the Alpini corps.

In 1937 he was then given command of the 2nd Cavalry Division Emanuele Filiberto Testa di Ferro and later that of the Army Corps of Padua.

[1][2] In 1938 he was promoted to Lieutenant General for exceptional merits, and in the same year he was sent to Italian East Africa where he assumed the post of Chief of Staff of the armed forces in the colony, as well as serving as deputy governor under Pietro Badoglio.

He was still serving in this capacity, now under the Duke of Aosta, when the Second World War broke out, and commanded Italian forces during the East African campaign.

In May 1941, at the end of the siege of Amba Alagi, he surrendered to the British along with the Duke and Generals Luigi Frusci, Pietro Pinna Parpaglia and Marino Valletti-Borgnini.