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.

After his father died in 1882 his mother Elizabeth changed her name to Macarthur-Onslow, and in 1887 she took her children to the United Kingdom to complete their education while she studied dairy farming.

[2] In 1900 Macarthur-Onslow made his own way to South Africa in 1900 to participate in the Second Boer War,[2] where he served as aide-de-camp to Hutton, who was now a major general.

[4] He returned to Australia in March 1901,[3] but was later appointed to command the 5th Battalion, Australian Commonwealth Horse.

[5][8] He was nominated to the New South Wales Legislative Council in 1922, then a lifetime appointment, and served there until 1933, but was largely inactive.

His other appointments included president of the Australian Club and director of the Royal Prince Alfred Hospital 1909 until 1929.