William Knox Peacock was born on 24 January 1898 in North Melbourne.  He was the younger son of Robert Knox Peacock and Emily Jane Finlay.  Robert was an Edinburgh trained doctor, and was a brother of Dr Samuel Peacock, famously charged with murder in 1911 after a young female patient disappeared from his premises at 62 Wellington Parade, East Melbourne.  It was presumed she had died as the result of an 'illegal operation'.  He was found guilty in spite of the body never being found; he appealed but the jury failed to agree and in a third trial the jury remained doubtful and he was acquitted.

When William enlisted on 7 May 1918 he gave his address as 62 Wellington Parade which was also the address of his next of kin, his sister, Ethel.  Both his parents at the time of his enlistment had died.  Robert died in 1899 and Emily, who remarried, died in 1914 when William was only sixteen and Samuel had taken on the role of guardian.

William by 1918 was a first year medical student at Melbourne University.  He had had previous military experience of four years with the Senior Cadets 69B, South Yarra, and with the Citizens' Forces Melbourne University Rifles for three years.  His papers were endorsed with the words, 'has passed the Light Horse Riding Test, and has volunteered for service in Egypt'.  He was appointed to Recruit Depot Battalion at Broadmeadows as a private.

He embarked on 16 October 1918 on the Malta and disembarked at Suez  on 22 November.  In Egypt he was allotted to the 4th Light Horse Regiment.  The Turks surrendered on 30 October and long serving troopers began to embark for home, but in March 1919 a revolt erupted in Egypt and William would have been involved in this. The riot was quelled within a month due largely to the 'flexibility and ability of the ANZAC forces'.  He returned with his regiment to Australia on 15 June 1919 from Kantara and was discharged on 8 August the same year.

After the war William completed his medical degree and moved to Perth where he married Lulie Rodda in 1929.  He died in 1992.