During the Mexican–American War in 1847, Reno commanded an artillery battery under General Winfield Scott and fought in the Siege of Vera Cruz and other battles in Mexico. Reno was brevetted twice during the war—once for "gallant and meritorious conduct" at the Battle of Cerro Gordo, and later for bravery at the Battle for Mexico City and the Battle of Chapultepec, where he was seriously wounded while commanding a howitzer battery. During the occupation of Mexico City, Reno became an original member of the Aztec Club of 1847.