Born at Larne, Co Antrim about 1880 the son of John Moore Killen MD, he was educated at Queen's College, Belfast and graduated in arts in the Royal University of Ireland, winning first-class honours and an exhibition; he proceeded to his medical degrees in 1903. After postgraduate study at the London Hospital, he was house surgeon at the Bolingbroke Hospital, Wandsworth, and took the Fellowship in 1908. He served as house surgeon and anaesthetist at the Royal Victoria Hospital, Belfast, and was demonstrator of anatomy at Queen's College. Killen practised for most of his life at Larne, and succeeded his father as medical officer to the Lame District and Fever Hospitals. He was also Admiralty Surgeon and Agent there, and was a Fellow of the Ulster Medical Society. He died on 21 January 1951.