He was Fellow of Trinity College, Cambridge from 1944–1948, but left England to return to his native India, where he served in the Indian Foreign Service until 1961. During that period he served on the Indian Delegation to the United Nations, working extensively with UNESCO and UNICEF. Leaving his diplomatic career to return to academe, Rajan taught at the University of Delhi before emigrating to Canada to take up a position at the University of Western Ontario.
Rajan's scholarly work covers a wide range of English poetry, but has returned frequently to Milton and particularly to Milton's Paradise Lost. He cannot be easily assigned to any critical methodology, but is instead a scholar of poetics in many forms and from many approaches. His 1947 book Paradise Lost and the Seventeenth Century Reader is primarily a response to Milton's apparent interest in Arianism, considered a heresy, and argues for a distinction between private and public meaning in Milton's poetry. The book was influential for William Empson, particularly Empson's critique of strictly theological readings of Paradise Lost, Milton's God. Later essays explore what Rajan calls "generic multeity" in Paradise Lost. In addition to his work on Milton, Rajan's later criticism addresses issues of meaning, intention, and context in a broad array of writers including Spenser, Yeats, Marvell, Keats, and Macaulay.
Rajan has also written two novels. The Dark Dancer is a sobering study of the conflicts of the Partition; Too Long in the West, on the other hand, is a more light-hearted satire (perhaps influenced by Tagore's Farewell, My Friend) about a girl's return to her home village after an emancipating education in New York.