James was the second son of James and Annie Horn of Wallsend NSW. His birth was registered at Wallsend in 1892. At the age of 24 he enlisted at Newcastle in January 1916. He had been teaching at Stanborough Public School, near Inverell and was a keen Rugby Union player.
James joined the 35th Battalion, 1st Reinforcement and departed from Australia on the transport ship HMAT A24 Benalla on 1 May 1916. The Battalion arrived in England in July. James was stationed at Tidsworth until he was sent to France in August 1917. He was wounded on 14 October 1917 during the Battle of Passchendale and died of his wounds the following day.
Corporal James Horn was buried at Lijssenthoek Military Cemetery, Belgium. His personal possessions were returned to his family and included a housewife, song book, pipe, knife, fountain pen, diary, gold ring and silver wrist watch. His name is inscribed on the Inverell Honor Roll and the Cenotaph and the Tingha War Memorial. In 1919 he was one of the men for whom a memorial tree was planted in Kurrajong Parade, Inverell.