He traveled to Galveston in January 1842, where he served in Capt. Jesse Billingsley's company during the Adrián Woll invasion. He then enlisted in the Somervell expedition and took part in the actions around Laredo and Guerrero. He also joined William S. Fisher's Mier expedition. Walker escaped at Salado, was recaptured, and survived the Black Bean Episode. In 1844 Walker joined John C. Hays' company of Texas Rangers and participated in the battle of Walker's Creek near the junction of Walker's Creek and West Sister Creek northwest of present-day Sisterdale in Kendall County. During the engagement the rangers, using new Colt revolvers, successfully defeated about eighty Comanches.
When Gen. Zachary Taylor requested volunteers to act as scouts and spies for his regular army, Walker enlisted as a private and was mustered into federal service in September 1845. In April 1846 he formed his own company for duty under Taylor. On April 28 Walker was ambushed with his company en route to join Taylor at Port Isabel. He reached Taylor's camp on April 29; his reports caused Taylor to move his encampment. Walker performed exemplary duty as a scout and courier on numerous other occasions. His company was the only Texas unit at the battles of Palo Alto and Resaca de la Palma. He was presented a horse by the grateful citizens of New Orleans in the spring of 1846 for his numerous exploits with Taylor's army.
Walker served as captain of the inactive Company C of the United States Mounted Rifles until the outbreak of the Mexican War. When the First Regiment, Texas Mounted Riflemen, was organized in June 1846, Walker was elected lieutenant colonel. He fought in the battle of Monterrey in September and on October 2, 1846, mustered out of federal service, activated his commission as captain of the mounted rifles, and proceeded to Washington, D.C., to begin recruiting for his company. During his recruitment excursion Walker visited Samuel Colt. Colt credited Walker with proposed improvements, including a stationary trigger and guard, to the existing revolver. The new six-shooter was named the Walker Colt.
After arriving with his new company at Vera Cruz, Mexico, Walker was detailed on May 27, 1847, to the First Pennsylvania Volunteers, stationed at Castle San Carlos de Perote to counter Mexican guerrilla activities between Perote and Jalapa. On October 5, 1847, Walker left Perote with Gen. Joseph P. Lane to escort a supply train to Mexico City. According to J. J. Oswandel, author of Notes on the Mexican War, who wrote about the incident, Walker grew increasingly embittered against the enemy: "Should Captain Walker come across guerillas, God help them, for he seldom brings in prisoners. The captain and most all of his men are very prejudiced and embittered against every guerilla in the country." En route Lane was informed of a sizable enemy force at Huamantla and ordered an attack. With Walker's mounted rifles in the lead, the assault force reached Huamantla on October 9. During the spirited contest that followed Walker was either shot in the back or killed by a man on foot carrying a lance. Following his death his unit took revenge on the community of Huamantla. Walker was buried at Hacienda Tamaris. In 1848 his remains were moved to San Antonio. On April 21, 1856, as part of a battle of San Jacinto celebration, he was reburied in the Odd Fellows' Cemetery in San Antonio.
29° 25.277, -098° 28.182
Odd Fellows Cemetery