James Headlam was born 24 December 1863, the 2nd son of the Reverend Canon Arthur Headlam, of Whorlton Hall, Barnard Castle, and his wife Agnes (Agnes died while James was still a boy, and his father remarried, to Louisa Woodall, in 1876). He was educated at Eton College, then King’s College, Cambridge and the University of Berlin [Germany]. In 1893 he married the pianist and composer Elisabeth (commonly known as Else) Sonntag, youngest daughter of Dr August Sonntag of Lüneburg; they had one son, Kenneth and one daughter, Agnes. In 1918, after the death of a Morley relative who had left him a family estate, Headlam received royal licence to assume the name and arms of Morley, and became James Headlam-Morley.

Headlam-Morley's career began as an academic, as a Fellow of King’s College, Cambridge (1890-1896), also lecturing for the Cambridge University Extension and holding the position of Professor of Greek and Ancient History at Queen’s College, London (1894-1900). He was also an honorary assistant commissioner to the Royal Commission on Secondary Education, Staff Inspector of Secondary Schools for the Board of Education (1902-1920) and a Member of the Prime Minister’s Committee on Modern Languages (1917-1918). During the First World War he worked in the War Propaganda Bureau (1914-1917), before becoming Assistant Director of the Political Intelligence Bureau in the Department of Information (1917-1918). At the end of the war Headlam-Morley continued his political work as Assistant Director of the Political Intelligence Department of the Foreign Office (1918-1920) and was a Member of the Political Section of the British Delegation to the Peace Conference at Paris [France] in 1919, before becoming Historical Adviser to the Foreign Office (1920 onwards). He was awarded the CBE in 1920, and was knighted in 1929. He died on 6 September 1929.

His publications include: On Election by Lot at Athens; Bismarck and the Foundation of the German Empire (1899, reissued 1926); Studies in Diplomatic History, (1930); on Classical Studies in Germany (Special Reports issued by the Board of Education); articles on Austria-Hungary, Germany, etc., in 10th edition of the Encyclopædia Britannica; The History of Twelve Days; The German Chancellor and the Outbreak of War; The Issue; and numerous other pamphlets; British Documents on the Origins of the War: Volume xi., The Outbreak of War (editor), etc.