Born Claire Delavenay in London, she was educated at Newnham College, Cambridge. She became literary editor of the 'New Statesman' and also the 'Sunday Times'. She has written several noted biographies and her work has been recognised with the award of the 1990 James Tait Black Memorial Prize and the 1991 Hawthornden Prize for 'The Invisible Woman The Story of Nelly Ternan and Charles Dickens'.
In addition, her biography of Samuel Pepys won the Whitbread Book Award in 2002, the Rose Mary Crawshay Prize in 2003, the Latham Prize of the Samuel Pepys Club in 2003, and was also shortlisted for the Samuel Johnson Prize in 2003.
She married her first husband, Nicholas Tomalin, who was a prominent journalist but who was killed in the Arab-Israeli Yom Kippur War in 1973. Her second husband is the novelist and playwright Michael Frayn.
She is Vice-President of the Royal Society of Literature and of the English PEN (International PEN).