De Castro was the Massachusetts State flag bearer of Company I, 19th Massachusetts Infantry, an all volunteer unit. The unit participated in the Battle of Gettysburg at Gettysburg, Pennsylvania as part of the III Corps 3rd Brigade, U.S. Army under the command of Colonel Norman J. Hall.

On July 3, 1863, the third and last day of the battle, his unit participated in what became known as Pickett's Charge. Pickett's Charge was a disastrous infantry assault ordered by Confederate General Robert E. Lee against Major General George G. Meade's Union positions on Cemetery Ridge.

During the battle, De Castro attacked a confederate flag bearer from the 19th Virginia Infantry regiment, with the staff of his own colors and seized the opposing regiment's flag, handing the prize over to General Alexander S. Webb. General Webb is quoted as saying, "At the instant a man broke through my lines and thrust a rebel battle flag into my hands. He never said a word and darted back. It was Corporal Joseph H. De Castro, one of my color bearers. He had knocked down a color bearer in the enemy's line with the staff of the Massachusetts State colors, seized the falling flag and dashed it to me".[1] On December 1, 1864, De Castro was one of seven men from the 19th Massachusetts Infantry to be awarded the Medal of Honor.

After the war De Castro entered the regular army and served for a few years. De Castro married Rosalia Rodriguez and in 1882 moved to New York City. There he was an active member of the Phil Kearny Post, number 8 GAR. He was employed by the NY Barge Office when on May 8, 1892, he died in his home at 244 West 22nd Street. His funeral was held at the 18th Street Methodist Church and he was buried at Fairmount Cemetery (Section 2, Lot 300, Grave 2) in Newark, New Jersey.

Joseph H De Castro