Thomas Arnold was received into the Roman Catholic Church on 12 January 1856, which made him so unpopular in his job that he resigned and returned to England with his family. Mary Arnold had her fifth birthday about a month before they left, and she had no further connection with Tasmania. Thomas Arnold at first could earn but a precarious livelihood, and his eldest child spent much of her time with her grandmother. She was educated at various boarding schools, and at 16 returned to live with her parents at Oxford where her father had a history lectureship.
On 6 April 1872, she married T. Humphrey Ward, a fellow and tutor of Brasenose College. For the next nine years she lived at Oxford. 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.
Ward helped establish an organization for working and teaching among the poor and was one of the founders of the Women's National Anti-Suffrage League in 1908. In this latter vein, some of her writings were under the name "Mrs. Humphry Ward". She also worked as an educator in the residential settlements she founded. Mary Ward's declared aim was 'equalisation' in society, and she established educational settlements first at Marchmont Hall and later at Tavistock Place in Bloomsbury. This was originally called the Passmore Edwards Settlement, after its benefactor John Passmore Edwards, but after Ward's death it became the Mary Ward Settlement. The Mary Ward Centre continues as an adult education college.
In the summer of 1908 she was asked by George Nathaniel Curzon, 1st Marquess Curzon of Kedleston and William Cremer to become the first president of Britain’s Anti-Suffrage League. Ward agreed and took on the job creating editing the Anti-Suffrage Review. She published a large number of articles on the subject, while two of her novels, The Testing of Diana Mallory and Delia Blanchflower, were used as platforms to criticize the suffragettes. In a 1909 article in The Times, Ward wrote that constitutional, legal, financial, military, and international problems were problems only men could solve. However, she came to promote the idea of women having a voice in local government and other rights that the men's anti-suffrage movement would not tolerate.