He was born on the Isle of Wight, and was educated at Winchester and Corpus Christi College, Oxford. There he excelled at Classics and was made a fellow of Oriel in 1815. His appointment to the headship of Rugby, a famous public school, turned the school's fortunes around, and he is portrayed as a leading character in the novel, Tom Brown's Schooldays. In 1841, he became Regius Professor of Modern History at Oxford. He was one of the Eminent Victorians in Lytton Strachey's book of that name.
Arnold's son was the poet, Matthew Arnold, and his granddaughter, Mary Augusta Arnold, became a famous novelist under her married name of Mrs Humphry Ward.