According to her Wikipedia article, she was the daughter of Mary Constance Wyndham (seen to the right in this group portrait by Sargent). She married Herbert Asquith, son of PM Herbert Henry Asquith and his first wife, née Helen Kelsall Melland.
Her article in thepeerage links to a short biography with this: "...After 1914 Lady Cynthia's life was darkened by trouble. She lost two of her brothers, to whom she was devoted, in the war. Further, now that Herbert Asquith was in the army, he was unable to support his family. Lady Cynthia in 1918 accepted an appointment as private secretary to Sir J. M. Barrie [qv.]. She soon became responsible for running his whole social and domestic life. She went on with this until his death, for Asquith returned from the war with his health too much weakened to take up regular work. She also added to her income by freelance writing. During the next thirty years her publications included anthologies of ghost stories and children's tales, biographies of the Duchess of York (1928) and Princess Elizabeth and Princess Margaret (1937); two novels, The Spring House (1936) and One Sparkling Wave (1943), and a book of short stories What Dreams May Come (1951).
Barrie died in 1937 leaving her heir to the greater part of his fortune and the Asquiths left London to live first at Sullington in Sussex and afterwards at Bath where Herbert Asquith died in 1947. Lady Cynthia later returned to London. Meanwhile a play of hers about the Tolstoys, entitled No Heaven for Me, had been produced at the Little Theatre, Bristol, in 1947. She published three volumes of reminiscences, Haply I May Remember (1950), Remember and Be Glad (1952), and Portrait of Barrie (1954). A life of Countess Tolstoy, entitled Married to Tolstoy, was published posthumously in 1960 and Lady Cynthia's Diaries (1915-18) in 1968..."