William D. Rees was born around 1842 in Armstrong County, Pennsylvania. By the early 1860s, he was working as a laborer.
He enlisted in the Union army on September 24, 1861, and he mustered in as a private in Company B of the 103rd Pennsylvania Infantry. According to his service records, he was 5 feet, 7 inches tall, with brown hair and gray eyes. He married Mary D. Williams on October 8, 1861, and he attempted to obtain a release from his military contract. He reported on October 11, however, that “i have been with the lawyer and he told me that i could not get free without a great deal of trouble.” The following month, he confessed that he “much regret[ted]” enlisting in the army and “causing so much misery.” Nonetheless, he remained devoted to the Union. In November 1861, he enclosed a poem entitled “Prayer of the young Soldiers Wife,” which included the lines, “if it be thy will oh god / to take the life thou gave/ then let him die where stripes and stars / Oer his head shall wave.”
The regiment took part in the Battle of Seven Pines and the Seven Day’s Battles. He fell ill in July 1862, and he died of typhoid fever in Philadelphia, Pennsylvania, on August 25, 1862.