David Ormsby-Gore, 5th Baron Harlech

At the 1950 general election, he was elected Member of Parliament for Oswestry, which he remained until 1961. Under Prime Minister Anthony Eden he served briefly, from November 1956 to January 1957, as Parliamentary Under-Secretary of State for Foreign Affairs; and under Prime Minister Harold Macmillan he was from 1957 to 1961 Minister of State for Foreign Affairs. After the election of U.S. President John F. Kennedy he was appointed British Ambassador to the United States on 18 October 1961.[4] This meant that he had to take the Chiltern Hundreds on 1 June,[5] so that he could resign from the House.

Ormsby-Gore knew Kennedy well from his time in London, where his father Joseph P. Kennedy had served as American Ambassador. Like Macmillan, Ormsby-Gore was distantly related to Kennedy, but had a closer relationship than did Macmillan with the President-elect and his brother Robert. Six months after Kennedy took office Ormsby-Gore was in Washington, D.C. Referred to under the Kennedy administration as "our kind of Ambassador", he supplied Kennedy with a stream of advice and Cuban cigars via his diplomatic bag. He was almost a resident at the White House, being more a friend of the family than a mere ambassador. After President Kennedy's assassination there were rumours of a romance between Ormsby-Gore and Jacqueline Kennedy. In 1968 he proposed marriage to her, but, she did not accept.[6] Ormsby-Gore was one of the pallbearers at Robert Kennedy's funeral along with Robert McNamara, John Glenn, W. Averell Harriman, C. Douglas Dillon, Kirk Lemoyne Billings (schoolmate of John F. Kennedy), Stephen Edward Smith (husband to Jean Ann Kennedy), David Hackett, Jim Whittaker and John Seigenthaler Sr. Under the Lyndon B. Johnson administration relations were more formal but remained excellent; and Ormsby-Gore maintained his position after the Labour government took power in Britain in 1964.

A fierce opponent of oil-barrel politics, Ormsby-Gore's terse dismissal of the phenomenon ran: "It would indeed be a tragedy if the history of the human race proved to be nothing more than the story of an ape playing with a box of matches on a petrol dump." The extent of his influence over the Kennedy administration is disputed. Unable to persuade the American government to agree with the British line over Yemen and the Congo, or to proceed with either a negotiated settlement with Soviet premier Nikita Khrushchev over Berlin or the Skybolt ballistic missile programme, he nevertheless played a significant role in the Cuban Missile Crisis and ensured that Britain's views were taken into account by the American government.

The friendships of Ormsby-Gore and Macmillan with John Kennedy helped secure the first Test-Ban Treaty in 1963. Macmillan and Ormsby-Gore had been attempting to achieve a test-ban treaty with the Russians for the past ten years, and won Kennedy over through letters from Macmillan and frank discussions between Ormsby-Gore and Kennedy. They convinced him to act like a statesman and conclude Test-ban treaties with Russia and not fear being branded as an appeaser by political opponents in the United States.

Ormsby-Gore was a participant in what is referred to as a "twenty-five year conversation to do with the role of a leader in a democratic society". He encouraged Kennedy to remain focused on issues relevant to the world and the future, rather than attempting to protect himself politically.

According to the Duchess of Devonshire, who travelled with the British delegation to Kennedy's funeral in November 1963, Macmillan's successor as Prime Minister Alec Douglas-Home had wanted to appoint Ormsby-Gore as Foreign Secretary, but R. A. Butler had insisted on having this post as a condition of serving under Home.[7]

Ormsby-Gore retired as ambassador in 1965, a year after his father died, and took his seat in the House of Lords as Lord Harlech, briefly also holding the position of deputy chairman of the Conservative Party. He also had a successful career as a television executive, founding HTV, and served as president of the British Board of Film Classification. He had an active interest in the avant-garde, and for nearly ten years, beginning in 1969, was patron of the Institute for Research in Art and Technology.

Lord Harlech was seriously injured in a car crash at Montford Bridge near Shrewsbury on the evening of 25 January 1985 and died at the Royal Shrewsbury Hospital the following morning, aged 66.[19] Senator Edward Kennedy, Jacqueline Onassis and other Kennedy family members attended his funeral in Llanfihangel-y-traethau.[20] He was succeeded in the barony by his second and only surviving son, Francis.

Ormsby-Gore was appointed to be a Deputy Lieutenant of Shropshire on 12 April 1961.[21] As the British Ambassador to the United States, he was, on 29 June 1961, appointed to the Order of St Michael and St George as a Knight Commander (KCMG).[22] In 1962, he was appointed to the Order of St John as a Knight (KStJ).[23]