When the Mexican war began Walker was the captain of a company of Texan Rangers stationed on the Rio Grande, and immediately offered his services to General Taylor, who accepted them, and stationed him between Point Isabel and the cantonment for the purpose of keeping open the communication. On the 28th of April he discovered that the Mexican troops were in motion, and at once, with his small command of twenty-five men, set out to report the fact to the general. On his way he encountered the Mexican column, and it is not improbable that with his small party he was in contact with one wing of the force which subsequently fought at Palo Alto. The Texans were pursued to Point Isabel, on which place they fell back, having lost several men, but killed more of the enemy than their own force numbered. In spite of the intervening force of the enemy, Walker determined to reach General Taylor on that night, and accompanied but by six of his men set out. After charging through a large body of Mexican lancers, he reached Gen. Taylor on the morning of the 30th. On the 1st of May Gen. Taylor broke up his camp, and what followed is well known. On the 3d Walker was again employed in the perilous service of ascertaining the condition of Fort Brown, which was then being bombarded by all the batteries of the city of Matamoras. His reconnoisance was one of the boldest feats performed during the war, and though May, who had command of a hundred horse for the purpose of covering him, presuming he must have been captured returned to Gen. Taylor, Walker again returned on the 4th, having accomplished his duty alone. At Palo Alto and La Resaca Walker again distinguished himself, and was mentioned by Gen. Taylor in the dispatch with the highest terms of commendation. For his distinguished services, on the organization of the Mounted Rifles, he was appointed a captain of cavalry in the regular service. After sharing in all the perils of the war, Walker devoted himself to the pursuit of the Guerilleros, who infested the road from Vera Cruz to the capital, and uniformly maintained his high reputation. In the affair of La Hoya, Sept. 20, 1847, he acted independently, and was perfectly successful.