James William Macarthur-Onslow was born on 7 November 1867 at Camden Park Estate, near Menangle, New South Wales, the son of Captain Arthur Alexander Walton Onslow and his wife Elizabeth née Macarthur, the granddaughter of wool pioneer John Macarthur.
Macarthur-Onslow read law at Trinity College, Cambridge, receiving Bachelor of Arts (BA) and Bachelor of Laws (LLB) degrees in 1890 [REF].