At Oxford, his Eton friendship with John Manners won him entree into a famous and fashionable circle of young aristocratic "bloods" and intellectuals known as The Coterie, including Patrick Shaw-Stewart, Raymond Asquith (son of the Prime Minister), Sir Denis Anson, Edward Horner, and most famously Lady Diana Manners, the most beautiful woman in England and the "Lady Di" of her day. He cultivated a reputation for eloquence and fast-living, and although he had established a reputation as a poet, he earned an even better reputation for gambling, womanizing and drinking in his studied emulation of the life of Charles James Fox. But his contacts with politicans such as Winston Churchill, H.H. Asquith and others were impeccable.
Following Oxford, he entered into the Foreign Service, and owing to the national importance of his work at at cipher desk, he was excluded from military service until 1917, when he joined the Grenadier Guards. He served with distinction as a Lieutenant in the campaigns of 1918, winning the DSO for conspicuous gallantry. Almost all of his closest friends, including Shaw-Stewart, Horner, Asquith and John Manners were killed in the war, drawing him closer to Lady Diana Manners, whom he married in 1920.
Returning to the Foreign Service, he became Principal Private Secretary to two ministers and played a significant role in the Egyptian and Turkish crises of the early 1920s before winning a seat in Parliament as a Conservative in 1924. He gave one of the most acclaimed maiden speeches of the century, and became known as a stalwart supporter of Stanley Baldwin, the Prime Minister, and a close friend of Chancellor of the Exchequer, Winston Churchill. He became a junior minister in the Treasury office in the late 1920s before losing his seat in the 1929 elections when the Conservative Party was swept out of office.
Turning to literature, he produced one of the most readable short biographies in the language, Talleyrand, which was published in 1932 to critical praise and has remained in print almost continuously since. He returned to Parliament in a by-election in 1931, and served continuously until 1945.
Returning to the Treasury Office as a junior minister, he was elevated to the Cabinet as War Secretary in 1935 and promoted to First Lord of the Admiralty in 1937. He completed a biography of Douglas Haig during this period. The most public critic of Neville Chamberlain's appeasement policy inside the Cabinet, he famously resigned in 1938 over the Munich Agreement with Adolf Hitler in an act that MP Vyvyan Adams described as "the first step in the road back to national sanity". He was active among the opponents of Neville Chamberlain and took a prominent role in the famous Parliamentary debate of 1940 which led to Chamberlain's downfall.
He subsequently entered the Cabinet as Minister of Information under Churchill, but surprisingly did not play a major role in the direction of the war until appointed the British Government's liason to the Free French in 1943. He subsequently became Ambassador to France in 1944, and was a great success in Paris. He left office in 1947, was knighted, and devoted himself primarily to literature until his death in 1954. He produced during this period the classic autobiography Old Men Forget, and was eventually created a viscount in 1952 in recognition of his political and literary career.
His only child, John Julius Norwich (1929-) became well known as a writer and television host, and his granddaughter Artemis Cooper has published several books. His wife Diana lived until 1981, producing a three-volume autobiography that was a best-seller in its own right, and was the subject of a biography by Philip Ziegler covering her career as a film and stage actress, as well as her life with Duff. Duff Cooper himself was the subject of a biography by John Charmley, and a major British literary award, the Duff Cooper Prize, was established in his name.