Jebb was educated at Eton College, then Magdalen College, Oxford, gaining a degree in History.
Jebb entered the Diplomatic Service in 1924. He served in Tehran, then at the Foreign Office in London where, amongst other positions, he served as the Private Secretary to the Head of the Diplomatic Service. During his thirty-six years as a diplomat he held posts such as assistant Under-Secretary in the Ministry of Economic Warfare, acting counsellor in the Foreign Office (1943), and head of the Reconstruction Department, all during World War II. During the war he also attended the Dumbarton Oaks Conference as a British delegate. Work at the conference led to the later formation of the United Nations, and Jebb would eventually become acting Secretary-General of the United Nations briefly in 1946. He was knighted in 1949. Other diplomatic work included being a British representative to the United Nations from 1950 to 1954 and serving as British Ambassador to France from 1954 to 1960. Later he presided over the North Atlantic Treaty Organization from 1963 to 1967. He also was the Liberal Party’s Deputy leader in the House of Lords from 1965 to 1988. From 1973 to 1976, he worked as a member of the European Parliament. Despite the demands of his career, he found time to write books, such as "The European Idea", "Half-Way to 1984", "Europe after De Gaulle", "Memoirs of Lord Gladwyn", and "Is Tension Necessary?". Jebb died on 24 October 1996 at the age of 96, and is buried at St. Andrew's, Bramfield, in the county of Suffolk.
When asked why he had joined the Liberal party in the early 1960s, he replied that the Liberals were a party without a general and that he was a general without a party. Like many Liberals, he passionately believed that education was the key to social reform.
In 1929, Gladwyn married Cynthia Noble, daughter of Sir Saxton Noble, 3rd Baronet, and granddaughter of Sir Andrew Noble, 1st Baronet, with whom he had one son and two daughters: Miles, Vanessa, married to the historian Hugh Thomas, and Stella, married to the scientist Joel de Rosnay. His granddaughter is the international best selling author Tatiana de Rosnay.