Arnold was born at Hobart, Van Diemen's Land, on 18 September 1852, eldest son and second child of Thomas Arnold; his mother was Julia, daughter of William Sorell, registrar of deeds, Hobart, and his elder sister was the novelist Mrs. Humphry Ward.
He helped to develop the literary section of the Guardian and he encouraged local artists, taking part in the establishment of the Manchester School of Art.
[1] He retired from the Manchester Guardian, due to spinal disease, in 1898, and the next year he moved to London, where he saw friends and wrote a little.
[1] A memorial volume by his sister Mary Augusta Ward and Charles Edward Montague, William Thomas Arnold, Journalist and Historian, was published in 1907.
Subsequently, in 'German Ambitions as they affect Britain and the United States' (1903), a collection of letters originally contributed to the 'Spectator' under the signature 'Vigilans et Æquus', Arnold proved his mastery of foreign contemporary literature and his ability to draw prudent deductions from it.
In 1886 he published a critical edition of the section on the Punic war in his grandfather's 'History of Rome' ; and contributions between 1886 and 1895 to the 'English Historical Review ' showed the strength of his interest in ancient history.
[1] In 1879 he won the Arnold prize with an essay on The Roman System of Provincial Administration to the Accession of Constantine the Great, which was published in 1879.
He was a contributor to Thomas Humphry Ward's English Poets (1880–2); and some dramatic reviews by him were published in The Manchester Stage, 1880–1900 (1900).
German Ambitions as they affect Britain and the United States (1903) was a collection of letters originally contributed to The Spectator under the signature "Vigilans et Æquus".