Mortimer continued in journalism, mainly for "The Sunday Times", and also wrote screenplays. Her biography of the Queen Elizabeth The Queen Mother was commissioned by Macmillan, but when completed it was rejected, and eventually published by Viking in 1986. Her former agent Giles Gordon, in his obituary of her in "The Guardian", called it "the most astute biography of a royal since Lytton Strachey was at work. Penelope had approached her subject as somebody in the public eye, whose career might as well be recorded as if she were a normal human being."