During the Mexican–American War, Borland was commissioned major of the Arkansas Mounted Infantry Regiment serving under Archibald Yell. Borland served throughout the war, having turned over his newspaper business to associates. He was taken as a prisoner of war by the Mexican army on January 23, 1847, just south of Saltillo. He escaped, and was discharged when his regiment was disbanded and mustered out in June, but continued in the army as volunteer aide-de-camp to General William J. Worth during the remainder of the campaign, from the Battle of Molino del Rey to the capture of Mexico City on September 14, 1847.