After the fall of the Alamo, Hill and his father set out with seven other men, including William Bennett Scates, to join Sam Houston's army. They met Capt. William W. Hill's detachment at Columbus, and young Hill joined this group while his father went on to army headquarters. Asa Hill was later detached from the army to warn settlers of Houston's planned withdrawal to the Brazos River. At San Jacinto, James M. Hill served in Company H of Col. Edward Burleson's First Regiment, Texas Volunteers. This company was led by Robert Stevenson in the absence of its commander, Captain Hill, who had contracted measles at Donahoe, on the Brazos. James Monroe Hill was one of the few men present at the first interview between Houston and Antonio López de Santa Anna and is portrayed in William Henry Huddle's painting The Surrender of Santa Anna, now hanging in the state Capitol.
Hill is said to have joined a volunteer company in response to Adrián Woll's invasion of Texas in 1842. Later that year his father and two brothers, Jeffrey Barksdale Hill and John C.C. Hill, participated in the Somervell expedition. Before John Hill left for the Rio Grande, his brother James gave him a new rifle, telling him "never to surrender it to a Mexican." After the battle of Mier, at which Jeffrey was wounded and all three of the Hills were captured, young John smashed his brother's gift against the pavement rather than give it up. Their father later drew one of the white beans at Saltillo in the Black Bean Episode.
On September 14, 1843, Hill married Jane Hallowell Kerr in Washington County. Jane was born in Tennessee in 1824, had immigrated to Texas with her family in 1831, and had taken part in the Runaway Scrape in 1836. The couple spent the next forty-one years in Fayette County. According to his wife's brief memoir, Hill was a soldier in the Confederate Army. The family moved in 1884 to Austin, where Hill opened a store on Congress Avenue. In April 1894 at a Texas Veterans Association meeting in Waco he was appointed chairman of a committee to locate the battlefield of San Jacinto. He and the other members of the committee, Francis R. Lubbock and William P. Hardeman, visited the ground on July 4 of that year and recommended that the state acquire the site as a memorial. In 1897 Governor Charles A. Culberson appointed Hill one of three commissioners to purchase the battleground. His colleagues on the committee were Sterling Brown Hendricks and Waller T. Burns. The purchase was made in November of that year.
In Austin, on October 19, 1897, Hill completed his recollections, "relating personal experiences and vivid details" of the battle of San Jacinto. He was then eighty years old and one of only ten surviving San Jacinto veterans. Upon the death of Guy M. Bryan on June 4, 1901, Hill, then first vice president (1893-1901), became president of the Texas Veterans Association. He was a Methodist and an honorary life member of the Texas State Historical Association. His papers, including a typescript of his reminiscences of San Jacinto, are preserved at the Barker Texas History Center, University of Texas at Austin.
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