Horatio Terrell was born around 1846 in Carmel, New York, to Ephraim and Mary Terrell. His father was a farm laborer who owned $50 of personal property by 1860. Terrell grew up in Carmel.
He enlisted in the Union army on September 6, 1861, and he mustered in as a private in Company C of the 48th New York Infantry four days later. He apparently lied about his age, claiming that he was 19 years old. According to his service records, he was 5 feet, 6 inches tall, with auburn hair and blue eyes. The regiment was stationed in South Carolina, and he was wounded in the Battle of Fort Wagner on July 18, 1863. As he explained to his brother, he was “wounded in the left leg, a rifle ball passed through” but did “not break any bones.” He was promoted to corporal on November 10, 1863. He deserted on February 28, 1864.