Agnes Headlam-Morley was born on 10 December 1902, the younger child of James Headlam [later Headlam-Morley] and his German wife Else. Agnes was educated at Wimbledon High School (Girls Public Day School Trust) and studied history at Somerville College, University of Oxford.

She became a Fellow and Tutor in politics at St Hugh’s College, Oxford, in 1932, and was also adopted as a prospective Conservative Candidate for the Barnard Castle Division of Durham, in 1936 (having previously acted as research worker and election helper for her cousin, Cuthbert Headlam, who was MP for the same seat until 1935). Agnes remained a Fellow at St Hugh's for nearly 40 years, retiring in 1970, and in 1948 became the Montague Burton Professor of International Relations (the first woman to hold a full professorship at Oxford). She was also an honorary fellow of Somerville College, Oxford, (1948), a Member of St Antony’s College and also of the Academic Council, Wilton Park. She was a founder member of the Anglo-German Association, and was received into the Roman Catholic Church in 1948. Agnes Headlam-Morley died on 21 February 1986.

Though active in teaching, Agnes's publications are mostly editions of her father's work. They include: "The New Democratic Constitutions of Europe" (her thesis, published in 1929); "Studies in Diplomatic History", by Sir James Headlam-Morley, which she edited with her brother Kenneth Headlam-Morley (1930); "Arthur Cayley Headlam" (a memoir published in The Fourth Gospel as History by A. C. Headlam, 1948); "Last Days", 1960; "A Memoir of the Peace Conference of Paris 1919" by Sir James Headlam-Morley, which she edited (1972); an essay on Gustav Stresemann in The History Makers, edited by Sir John Wheeler-Bennett and Lord Longford (1973); a contribution to the Longford Report on Pornography (1976); articles and reviews in Trivium and History Today.