The patroness of Millesimo is the Madonna del Carmine, celebrated on 16 July. The town and its castle were walled by Enrico II Del Carretto, marquis of Finale, at the crossing of the Bormida river by the Ponte della Gaietta, a strategic point and a source of toll collection toward Cortemilia and the port of Finale. The castle, once belonging to the Del Carretto marquises and municipal property since 1882, was the scene of a particular episode in 1809: inside it, on 17 August of that year, Pope Pius VII, a prisoner of Napoleon Bonaparte, celebrated Mass on an altar still kept in the council chamber.
On 13 April 1796 Millesimo was at the centre of the famous Battle of Cosseria, in which Napoleon Bonaparte defeated the Austro-Sardinian troops after a fierce clash that cost more than three thousand soldiers their lives: at the nearby castle of Cosseria, Colonel Filippo del Carretto fell heroically, defending it at the head of his Piedmontese grenadiers. Millesimo's territory is also home to the monastery of Santo Stefano, donated in 1216 to Cistercian nuns after its purchase by the same Enrico II Del Carretto.
