Cynthia Jebb was born on 20 November 1898, the youngest of the four children of Sir Saxton Noble, and his wife, Celia. In 1912 she went to Northlands, a small girls' school at Englefield Green, Surrey, where she was mainly taught in literature and music, and at the end of the war, in which her elder brother Marc was killed, she came out into society. On 22 January 1929 she married the young diplomat [Hubert] Gladwyn Jebb, having a son, Miles [later 2nd Lord Gladwyn], and two daughters, Vanessa and Stella.

In 1932 Cynthia accompanied her husband to Rome, where he had been posted as a secretary in the British embassy. In 1935 they returned to London, where over the next fifteen years Jebb's diplomatic career kept him in the Foreign Office, before a key posting to New York as the British representative on the Security Council of the United Nations. Cynthia's talents as a hostess were invaluable, both in New York, and even more so in Jebb's next (and final) posting as British Ambassador to France (1954-60), where she ran the Embassy, managing the arrangements for several highly successful royal and prime ministerial visits to Paris at a key time in British relations with France.

Jebb retired from the diplomatic service in 1960, becoming 1st Lord Gladwyn, and as Lady Gladwyn, Cynthia continued to pursue an active social life, particularly in the world of music, also publishing a book on the Paris Embassy in 1976. She had kept a diary throughout much of the post-war era, giving her frank and vivid opinions of many of the social and political figures of the day, and these diaries, edited by her son, were published in 1996. Cynthia Gladwyn died on 21 September 1990.