Garland was promoted to lieutenant colonel of the 4th U.S. Infantry on May 7, 1839. He fought under Zachary Taylor at the battles of Palo Alto and Resaca de la Palma. During this time he commanded a second lieutenant U.S. Grant. For services in these two battles, he was brevetted to full colonel and commanded a brigade of regulars in David E. Twiggs' division of the Army of Occupation. At the Battle of Monterrey, Garland temporarily led the division in the initial attack on the city as General Twiggs had taken an overdose of a laxative before the battle.