Kathleen Mary Constable was the daughter of a Yorkshire journalist and was brought up a Quaker. She was educated at Ackworth school and at the Mount School in York. She was a brilliant student at Somerville College, Oxford, where she won the Charles Oldham Shakespeare Scholarship in 1926. After graduation, she was appointed as a tutor at Somerville and St. Hilda's College before moving to Bedford College, London University.
There she became a lecturer in 1939, a reader in 1947 and, in 1958, was named the Hildred Carlile Professor of English, a position she held until her retirement in 1971. In 1933, she married Geoffrey Tillotson, later a Professor of English at Birkbeck College. Together they published a volume of essays, Mid-Victorian Studies (1965) and an annotated edition of Thackeray's Vanity Fair (1963). Kathleen Tillotson's independent writing career had begun with a work on Michael Drayton, which earned her the British Academy's Rose Mary Crawshay Prize in 1943. She then shifted her interest to the Victorians, and became one of the pre-eminent experts on 19th-century literature, as well as one of the most distinguished scholars of her generation. Her book Novels of the Eighteen-Forties (1954) is now considered a classic. Dickens at Work (1957), on which she collaborated with John Butt, examined Dickens's working methods for the first time. One of Prof. Tillotson's great achievement was the Pilgrim Edition of The Letters of Charles Dickens, beginning in the early 1960s. She acted as associate editor of volumes two and three, and as editor or joint editor of the five succeeding volumes. Her meticulous research was acknowledged by her being named as consultant editor for the ninth volume, which appeared in 1997.