It was named after the Marengo plain near Alessandria to commemorate the eponymous French victory.

It was formed in 1802, detaching part of the department of Tanaro, when the Subalpine Republic (formerly the mainland portion of the Kingdom of Sardinia) was directly annexed to France.

Initially it comprised the former Piedmontese provinces of Alessandria, Casale, Tortona, Voghera and Bobbio.

At the Congress of Vienna, the Savoyard King of Sardinia was restored in all his previous realms and domains, including Piedmont.

The department was subdivided into the following arrondissements and cantons (situation in 1812):[1] Its population in 1812 was 318,447, and its area was 348,261 hectares.