John Knox Peacock was born on 14 May 1896 at North Melbourne.  He was the elder son of Robert Knox Peacock and Emily Jane Finlay.  Robert was an Edinburgh trained doctor, and was a brother of Dr. Samuel Peacock, famously accused of murder in 1911 after a young female patient disappeard from his premises at 62 Wellington Parade, East Melbourne.  It was presumed she had died as the result of an 'illegal opeation'.  He was found guilty in spite of the body never being found; he appealled, but the jury failed to agree; and in a third trial the jury remained doubtful and he was acquitted.

When John enlisted on 4 April 1917 he gave his own address as Cranbourne where he was working as a farmer, but he gave his younger brother, William Knox Peacock, of 62 Wellington Parade, as his next of kin.  Both his parents at the time of his enlistment had died.  Robert died in 1899 and Emily, who remarried, died in 1914. Samuel had taken on the role of guardian to the young family and, as such, signed John's papers giving his permission for John to join the war.

John had had previous military experience of four years with the Senior Cadets 69B.  Confusingly it is a note from the area officer 64B at the Drill Hall, Powlett Street, East Melbourne who provides confirmation of John's date of birth.  He was initially appointed to the Field Artillery 31st Reinforcements at Maribyrnong as a gunner but after spending three weeks in isolaton camp in Ascot Vale with mumps he was transferred to the 34th Reinforcements.  He embarked on 22 December 1917 aboard the Ulysses and arrived at Southampton on 15 February 1918 via train and ship through Suez, Alexandria, Taranto and Cherbourg.  He proceeded overseas to France on 24 Aprl 1918 and from General Base Depot he was taken on strength of 3rd Field Artillery Brigade on 1 May 1918.

He returned to Asutralia on 9 July 1919 and was discharged on 28 September the same year.