The eldest child of Valma and Eric Stockton, she was named after Lorna Doone [1]. Sage was born at Hanmer, Flintshire, Wales, and educated at the village school, then at the Girls' High School in Whitchurch, Shropshire. Her childhood in the late 1940s and early 1950s is recalled in her last book Bad Blood. Sage became pregnant when she was 16 but was able to continue her education and won a scholarship to read English at Durham University, only after the university changed its admission rules to allow married couples to study there. Sage went on to receive an MA from Birmingham University for a thesis on seventeenth century poetry.

All of her academic career was spent at the University of East Anglia, where she was Professor of English Literature from 1994. She edited The Cambridge Guide to Women's Writing in English (1999) which has become a standard work. In the Preface she wrote: "In concentrating on women's writing...you stress the extent and pace of change, for the scale of women's access to literary life has reflected and accelerated democratic, diasporic pressures in the modern world".