Trochu was born at Le Palais on the island of Belle-Île-en-Mer, off the coast of Brittany. Educated at St. Cyr, he received a commission in the Staff Corps in 1837, and was promoted to lieutenant in 1840, and captain in 1843. He served as a captain in Algeria under Marshal Bugeaud, who, in recognition of his gallantry in the battles of Sidi Yussuf and Isly, made him his "aide-de-camp" and entrusted him with important commissions. He was promoted to major in 1845, and to colonel in 1853. He served with distinction throughout the Crimean campaign, first as "aide-de-camp" to Marshal St. Arnaud, and then as general of brigade, and was made a commander of the Légion d'honneur and general of division. He again distinguished himself in command of a division in the Italian campaign of 1859, where he won the Grand Cross of the Légion d'honneur.