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. He was educated at Sydney Grammar School. After his father died in 1882 and 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.
Macarthur-Onslow read law at Trinity College, Cambridge, receiving Bachelor of Arts (BA) and Bachelor of Laws (LLB) degrees in 1890. He returned to Australia in 1891. He married Enid Emma Macarthur, the granddaughter of Hannibal Hawkins Macarthur in 1897. - James' great-grandfather and Enid's great-grandfather were brothers.
Macarthur-Onslow was commissioned captain of the Camden Squadron of the New South Wales Mounted Rifles in 1892. In 1894 he was selected by commandant of the military forces in New South Wales, Colonel Edward Hutton, for training in India with the 11th Hussars, the Royal Artillery, and the 1st Battalion, King's Royal Rifle Corps of the British Army. He served in India from 1894 to 1895, seeing active service in the Chitral Expedition.
In 1900 he made his own way to South Africa in 1900 to participate in the Boer War, where he served as aide-de-camp to Hutton, who was now a major general. He participated in operations in the Orange Free State from February to May 1900, including the actions at Vet River and Zand River, and operations in the Transvaal from May to November 1900. For this service, he was mentioned in despatches by Field Marshal Lord Roberts. He returned to Australia in March 1901, but was later appointed to command the 5th Battalion, Australian Commonwealth Horse.
He served as aide-de-camp to the Governor-General of Australia, the Earl of Hopetoun in 1902. He commanded the 2nd Light Horse Regiment from July 1903 until December 1907 when he was promoted to colonel on assuming command of the 1st Light Horse Brigade. He was on the unattached list in January 1910. Although too old for active service during the Great War, Macarthur-Onslow held the rank of Colonel with the Sea Transport Service of the First Australian Imperial Force, commanding troop ships travelling between Australia, the Middle East and Britain between 1915 and 1917. He was aide-de-camp to the Governor-General a second time from 1917 to 1920. He retired with the rank of major general in 1924.
Macarthur-Onslow served the people of New South Wales as a member of the New South Wales Legislative Assembly, representing the seats of Waverley from 1907 to 1913, Bondi from 1913 to 1920, and Eastern Suburbs from 1920 to 1922. 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.
Macarthur-Onslow served as a director of The Commercial Banking Company of Sydney Limited and The Colonial Sugar Refining Company Limited. His other appointments included president of the Australian Club and director of the Royal Prince Alfred Hospital 1909 until 1929.
Macarthur-Onslow died at Camden Park and was survived by his wife, his son and his two daughters. His papers are in the Mitchell Library, State Library of New South Wales.
A small selection of other notable family members includes:Father
Captain Arthur Alexander Walton Onslow (1833–1882) - father
Arthur Pooley Onslow - Arthur Alexander's father
Rosa Roberta, née Macleay - Arthur Pooley Onslow's wife (married 1832) - Arthur Alexander's mother
Their children, Arthur Alexander's siblings, included:
Sir Alexander Campbell Onslow (1842–1908)
Alexander Macleay (1767–1848) - Rosa Roberta's father - Arthur Alexander's grandfather. Fathered 17 children, including:
William Sharp Macleay (1792–1865) He never married and the heir to his estate was his brother, George W. J. Macleay
Sir George Macleay (1809–1891)
James Robert (1811–1892) secretary of the commission for the suppression of the slave trade in Cape Colony.
Margaret (b. 1802) married Archibald Clunes Innes (1800–1857) in 1829
Christiana Susan (b.1799) married Captain William Dumaresq (1793–1868) in 1830
Barbara Isabella (b.1797) married Pieter Laurentz Campbell (1809–1948) in 1834.
Sir William John Macleay (1820–1891) - Alexander Macleay's nephew
Elizabeth née Macarthur (1840–1911) - mother
Arthur and Elizabeth had 8 children: 6 boys and 2 girls - see siblings below
She inherited the estates of her childless uncles Sir Edward and Sir William, and that of her father James.
She changed her name to Macarthur-Onslow in 1892.
James Macarthur (1798–1867) - Elizabeth's father
John Macarthur (1767–1834) - Elizabeth's grandfather
Elizabeth Macarthur (1766–1850) - John's wife. Their other children included:
1. Sir Edward Macarthur (1789–1872) Married Sarah 1862. Died childless.
3. John - lawyer - remained in England
4. James (1798–1867)
5. Sir William Macarthur (1800–1882) Died unmarried.
Siblings (5 brothers, 2 sisters)
James William Macarthur-Onslow (1867–1946)
William died in WWI
Emily Susan (1869-1876)
Rosa Sibella Macarthur Onslow (1871–1943) Unmarried.
Brigadier General George Macleay Macarthur-Onslow (1875–1931) - brother
He had married at Manar, near Braidwood, Violet Marguerite Gordon on 16 October 1909. She and their daughter survived him.
(Francis) Arthur Macarthur-Onslow (1879–1938) - brother (served in the South African War)
He was survived by his wife Sylvia Seton Raymond, née Chisholm, whom he had married on 16 May 1903 at Goulburn, and by three sons and a daughter.
Major General Sir Denzil Macarthur-Onslow - Arthur's eldest son
Enid Emma Macarthur - wife
John Macarthur was James Macarthur-Onslow's great-grandfather
John's elder brother James was Enid's great-grandfather
Hannibal Hawkins Macarthur - Enid's grandfather - John Macarthur's nephewHannibal had 6 daughters and 5 sons
3. George Fairfowl Macarthur (1825–1890) 3 sons and 2 daughters
James Arthur (1898–1959) married Constance Herbert, disinherited and bankrupted by his father
Helen Maud (1899–1968) married Major General Sir Reginald Stanham and inherited Camden Park
Brigadier Richard Quentin Macarthur-Stanham - after her death it passed to her son
Elizabeth Enid (1903–1990) married Frederick Rothe