He entered the Mexican–American War as a first lieutenant and was cited for bravery and noteworthy service at the Battles of Contreras and Churubusco. His valor displayed at the Battle of Chapultepec earned for him a brevet promotion to the rank of captain. In March 1855 Palmer was promoted to the full rank of captain in the 2nd U.S. Cavalry.
Palmer entered the Mexican–American War as a first lieutenant and was commended for bravery and significant service at the Battles of Contreras and Churubusco. His valor during the Battle of Chapultepec led to an honorary promotion to captain. In March 1855, Palmer received his official promotion to captain in the 2nd U.S. Cavalry.