Sage was born in Hanmer — in Wales near the border with England — as the eldest child of Valma (née Meredith-Morris) and Eric Stockton, a haulage contractor, then serving as a second lieutenant in the Royal Welch Fusiliers.

Their marriage was deeply dysfunctional; her grandfather was an Anglican clergyman and a philandering alcoholic whose wife disliked him and lived in a separate area of their house.

Lorna later received an MA from the University of Birmingham for her thesis entitled "Poems on Poetry in the 17th Century".

[1][6] Sage spent her entire academic career at the University of East Anglia, where she became Professor of English Literature in 1994.

[6] The posthumous collection Moments of Truth partly consists of reprinted introductions to classic works.