Mary Augusta Arnold was born in Hobart, Tasmania, Australia, into a prominent intellectual family of writers and educationalists. Mary was the daughter of Tom Arnold, a professor of literature, and Julia Sorrell. Her uncle was the poet Matthew Arnold and her grandfather Thomas Arnold, the famous headmaster of Rugby School. Her sister Julia married Leonard Huxley, the son of Thomas Huxley, and their sons were Julian and Aldous Huxley. The Arnolds and the Huxleys were an important influence on British intellectual life.
Mary's father Tom Arnold was appointed inspector of schools in Van Diemen's Land (now Tasmania) and commenced his role on 15 January 1850. Tom Arnold was received into the Roman Catholic Church on 12 January 1856, which made him so unpopular in his job (and with his wife) that he resigned and left for England with his family in July 1856. Mary Arnold had her fifth birthday the previous month before they left, and she had no further connection with Tasmania. Tom Arnold was ratified as chair of English literature at the contemplated Catholic university, Dublin, after some delay. Mary spent much of her time with her grandmother. She was educated at various boarding schools, (at one period, from ages 11 to 15, in Shifnal, Shropshire) and at 16 returned to live with her parents at Oxford, where her father had a lecturership in History. Her schooldays formed the basis for one of her later novels, Marcella (1894).
On 6 April 1872, not yet 21 years old, Mary married Humphry Ward, a fellow and tutor of Brasenose College, and also a writer and editor. For the next nine years she continued to live at Oxford, at 17 Bradmore Road, where she is commemorated by a blue plaque. She had by now made herself familiar with French, German, Italian, Latin and Greek. She was developing an interest in social and educational service and making tentative efforts at literature. She added Spanish to her languages, and in 1877 undertook the writing of a large number of the lives of early Spanish ecclesiastics for the Dictionary of Christian Biography edited by Dr William Smith and Dr. Henry Wace.