Durand entered the Indian Civil Service in 1873. During the Second Anglo-Afghan War (1878–1880) he was Political Secretary at Kabul. From 1884 to 1894, he was Foreign Secretary of India. Durand was appointed Minister plenipotentiary at Tehran in 1894 although despite being a Persian scholar and speaking the language fluently he made little impression either in Tehran or on his superiors in London. He left Persia in March 1900, by which time owing to the illness of his wife Ella he had withdrawn from social life and the legation was in a depressed and disorganised state. From 1900 to 1903 he served as British Ambassador to Spain, and from 1903-1906 as Ambassador to the United States. He was appointed a CSI in 1881 knighted a KCIE in 1888 and a KCSI in 1894 and appointed a GCMG in 1900.
From 1906, after his return to England, he devoted his time to writing.
He also published the biography of his father, General Henry Marion Durand (1812–1871), and also had ambitions as a novelist, often with his wife, Lady E. R. Durand (1852–1913), as a co-author. Some of his publications are:
The Durand Line is named after Sir Mortimer and remains the international border between Afghanistan and modern-day Pakistan that is officially recognized by all countries apart from Afghanistan. The border is an ongoing point of contention between the two countries, as Afghanistan unilaterally disputes the legitimacy of the border.
In 1884 Durand informed Abdur Rahman Khan, the Amir of Afghanistan, the frontier between modern-day Pakistan (the successor state of British India) and Afghanistan that the garrison of Pandjeh had been slaughtered on the orders of the Russian General Komarov. The Russians wished to stop British occupation of Herat, so Durand was despatched to prevent "the strained relations which then existed between Russia and ourselves, wrote the Viceroy, Lord Dufferin, might in itself have proved the occasion of a long miserable war." Tensions at home in British newspapers heightened the urgency of the incident, threatening war in Central Asia, which Rahman was desperate to avoid. A telephone line was kept open between Lord Granville and Count Giers in St Petersburg.
Sir Mortimer was deputed to Kabul by the government of British India for the purpose of settling an exchange of territory required by the demarcation of the Joint Boundary Commission between northeastern Afghanistan and the Russian possessions along the same lines as in 1873, except for the southward salient at Pandjeh. The British made it clear that any further extension towards Herat would be amoun to a declaration of war. Rahman showed his usual ability in diplomatic argument, his tenacity where his own views or claims were in debate, with a sure underlying insight into the real situation. A Royal Commission was established to demarcate the boundary between Afghanistan and the British-governed India. The two parties camped at Parachinar, now part of FATA Pakistan, near Khost Afghanistan. From the British side the camp was attended by Mortimer Durand and Sahibzada Abdul Qayyum, the Political Agent for Khyber. The Afghans were represented by Sahibzada Abdul Latif and Governor Sardar Shireendil Khan representing Amir Abdur Rahman Khan. The territorial exchanges were amicably agreed upon; the relations between the British Indian and Afghan governments, as previously arranged, were confirmed. The Durand Road in Lahore is also named after him.
Durand died at Quetta, Baluchistan Agency, British India, in 1924. His grave yet to be discovered. However, his father is buried in a Church at Dera Ismail Khan, Khyber Pakhtunkhwa Province of Pakistan.
- John F. Riddick, The history of British India, p258
- "Court Circular". The Times (36085). London. 9 March 1900. p. 6.
- London Gazette, 1 March 1881
- London Gazette, 2 January 1894
- London Gazette, 1 June 1900
- The Liberal Year Book, 1907
- British Parliamentary Election Results, 1885-1918 FWS Craig
- Debrett's House of Commons & Judicial Bench, 1916
- Hopkirk, The Great Game, pp.430-1
- "Grave Sir Henry Mortimer Durand - Wikimapia". wikimapia.org. Retrieved 26 September 2016.
- Adye, Gen Sir John (1897). Indian Frontier Policy.
- Coen, T.C. (1971). The Indian Political Service.
- Durand, Col Algernon (1899). The Making of a Frontier Five Years' Experiences and Adventures in Gilgit, Hunsa, Nagar, Chitral and the Eastern Hindu - Kush.
- Marvin, Charles (1885). The Russians at the Gate of Herat.
- Riddick, John F. The history of British India.