After a short period as a teacher at Barnsbury Girls' School in Islington, north London (1961–1963), she has worked as a novelist, biographer and freelance literary critic, contributing regularly to book programmes on television, to BBC Radio 4 and various newspapers and magazines. She was a member of the BBC Advisory Committee on the Social Effects of Television (1975-1977), the Arts Council Literary Panel (1978-1981), and chief reviewer for non-fiction in the Evening Standard (1977-1980). She was elected a Fellow of the Royal Society of Literature in 1975.

Forster is married to the writer, journalist and broadcaster Hunter Davies. They live in London and in the Lake District.

She is the author of many successful novels, including Georgy Girl (1965) (filmed in 1966 and adapted for a short-lived 1970 Broadway musical), Lady's Maid (1990), Diary of an Ordinary Woman (2003), Have the Men Had Enough? (1989) and The Memory Box (1999), two memoirs, Hidden Lives (1995) and Precious Lives (1998), and several acclaimed biographies, most recently Good Wives (2001) and a fictionalised biography of the artist Gwen John, Keeping the world away (2006). She wrote Rich Desserts and Captain's Thin (1997), an account of the Carr's biscuit factory in Carlisle.

She has won awards for both her fiction and non-fiction works : Elizabeth Barrett Browning: a biography (Heinemann Award, 1989); Daphne du Maurier: The Secret Life of the Renowned Storyteller (Writers' Guild Award for Best Non-Fiction, 1993 - Fawcett Society Book Prize, 1994); Rich Desserts and Captain's Thin: a Family and Their Times 1831-1931 (Lex Prize of The Global Business Book Award, 1997); Precious Lives (J. R. Ackerley Prize for Autobiography, 1999).