Matthew McCarthy was born around 1841 in County Kilkenny, Ireland. He eventually immigrated to America. He enlisted in the Union army on January 2, 1866, and he mustered in as a sergeant in the 3rd United States Cavalry. According to his enlistment records, he was 5 feet, 5 inches tall, with brown hair and grey eyes. He eventually earned a promotion to sergeant. He mustered out on October 2, 1868, and he died sometime afterwards.