During the Spanish-American war, he joined the United States Army's 1st Volunteer Cavalry Regiment, known as the Rough Riders, where he held the rank of sergeant, and was considered by Theodore Roosevelt "one of the best non-commissioned officers we had".[2] Fish was a member of "L" troop commanded by Captain Allyn K. Capron Jr. He was not the only soldier from a prominent family in the unit, which contained a number of Ivy Leaguers from wealthy Eastern families. In citing their qualifications for active duty, Roosevelt touted their athletic accomplishments: Hamilton Fish was "the ex-captain of the Columbia crew"; Dudley Dean was "perhaps the best quarterback who ever played on a Harvard elevan"; Bob Wrenn was "the champion tennis player of America." Other college athletes included Yale high-jumper Edward C. Waller, steeplechase rider Craig Wadsworth, and polo-player Joe Stephens.[2]
Fish was one of the first Americans killed at the Battle of Las Guasimas, near Santiago, Cuba, on June 24, 1898. His final moments were described by one of his troopers, Ed Culver, who was wounded by the same bullet.[3] The journalist Burr McIntosh, who was present at the battle, saw his body shortly after his death and observed that "there was no sign of pain, only the faint suggestion of the old smile of victory, which I had so often seen".[4] Fish's death was widely covered in the press: his youth, his famous family, and his pathetic end made him one of the popular heroes of the war.[5] McIntosh, who had taken the last photograph of Fish alive, resisted the temptation to photograph the face of the man in death,[4] but he nevertheless took photographs of the covered body, alongside that of another fallen American soldier. First published in Leslie's Weekly on July 28, 1898, and often reproduced, these were among the very few published photos of American war dead, and became "one of the most enduring images of the Spanish-American war".[5]