In October 1932, Harrison was appointed as a third secretary in
His Majesty's Diplomatic Service, and in October 1937, he was promoted to second secretary. In July 1942, he was acting first secretary. As a junior diplomat at the Foreign Office, Harrison drafted a memorandum, "The Future of Austria", which greatly contributed to the notion of
Austria as an independent state. Harrison also contributed to the British draft declaration on Austria for the 1943
Moscow Declaration. He was also the principal drafter of Article XII of the
Potsdam Agreement, which concerned the
expulsion of ethnic Germans from central and eastern Europe after World War II. On 1 October 1956, Harrison was granted his first ambassadorship, as Ambassador Extraordinary and Plenipotentiary to Brazil. On 3 November 1958, he was transferred to Tehran as Ambassador to Iran/Persia. Between 1963 and 1965, Harrison was based in London as Deputy Under Secretary of State at the Foreign Office. On 27 August 1965, Harrison was appointed as ambassador to the Soviet Union. In 1968, he engaged in a brief affair with a Russian chambermaid who was working at the British Embassy. Harrison recalled not asking or knowing if she worked for the
KGB, but he said that it was assumed that every Soviet employee at the embassy worked or was an agent for the Soviet secret service. When security concerns arose over the
Soviet occupation of Czechoslovakia, and after he had been sent incriminating photographs taken by the KGB, Harrison informed the Foreign Office of his indiscretion, which immediately terminated his appointment and recalled him to Britain. Harrison revealed the affair to
The Sunday Times newspaper in 1981. The journalist and author
John Miller, who was part of the British press corps in the Soviet Union at the time of Harrison's ambassadorship, revealed more details of the affair in his memoir
All Them Cornfields and Ballet in the Evenings: Miller named the maid with whom Harrison was involved as Galya Ivanov and said he was told that by a Russian contact that she was not only a KGB agent but also the sister of
Eugene Ivanov, the Soviet naval attaché in Britain involved in the
Profumo affair. ==Honours==