She became a tutor at Oxford in 1941 and was a fellow of St Hilda's College, Oxford from 1942. In 1966, she became Merton Professor of English literature in the University of Oxford, the first woman to hold this chair. Her specialist areas were T. S. Eliot, the Metaphysical poets, Milton and religious poetry, with many essays published on the subjects, as well as on literary criticism itself. She edited The New Oxford Book of English Verse, 1250-1950 and The Metaphysical Poets.
Her 1949 collection of essays, The Art of T.S. Eliot, is regarded a seminal work on the poet. In particular, she challenged the notion that Eliot was only accessible to those well-versed in his many allusions:
"It is better, in reading poetry of this kind, to trouble too little about the ‘meaning’ than to trouble too much. If there are passages whose meaning seems elusive, where we feel we are ‘missing the point,’ we should read on, preferably aloud... We must find the meaning in the reading...She revisited Eliot's work in 1978 with her study of The Composition of Four Quartets, published by Faber and Faber.
Gardner also compiled The Faber Book of Religious Verse (1972) and The New Oxford Book of English Verse (1973).
Her work led to great acclaim, being awarded the CBE in 1962 and a Dame Commander of the Order of the British Empire in 1967, and receiving honorary degrees from Cambridge, London, Harvard, and Yale universities.