John William Seabrook enlisted with the Royal Australian Navy in March 1913, having spent time serving with the Royal Navy. He joined HMAS Sydney upon its commission in June 1913 and served on the ship for the entirety of the war. During this period, Seabrook saw action during the Sydney-Emden encounter on 9th November 1914, recording the event in great detail. He continued employment with the RAN until early 1919, when he was invalided out. After the First World War, John Seabrook became active in the Ex-Naval Men's Association, working as the organisation's honorary general treasurer.