William Arnold was born in 1852, the son of Thomas Arnold, an academic and his wife, Julia Sorrell. He was the grandson of Thomas Arnold, the reforming headmaster, and his uncle was Matthew Arnold. His sister, Mary, was better known as the author Mrs Humphrey Ward, and she later wrote a memoir of her brother.
Arnold attended University College, Oxford, graduating in 1876. In 1879 he joined the staff of the Manchester Guardian, where he wrote editorials, reviews and criticism. Arnold was an important figure at the paper, influencing its house style and its political opinions, particularly the paper's support for Irish Home Rule. He retired from the Guardian in 1899 and died in 1904 after a long illness.
Outside of journalism, Arnold was interested in the poetry of John Keats, and he published an edition of Keats' work in 1884. His great interest, however, was Roman history. Arnold published a number of articles and monographs in this area, including The Roman system of provincial administration to the accession of Constantine the Great, based on his Arnold Prize essay of 1879. At the time of his death, he had been working for many years on a major study of the Roman Empire. This was later edited for publication by his friend, Edward Fiddes, and published as Studies of Roman Imperialism in 1906. This work included memoirs of the author by Mrs Humphrey Ward and his Guardian colleague, C E Montague.
Arnold married Henrietta Wale in 1877; there were no children of the marriage.