Sir Patrick Reilly was born at Ootacamund, India on 17 March 1909, the only son of Sir D'Arcy Reilly, Indian Civil Service and Florence Reilly (née Wilkinson). He was educated at Winchester and New College, Oxford, where he obtained a double First in Hon. Mods and Greats. In 1932 with a career in the Diplomatic Service in mind he won a Laming Travelling Fellowship at the Queen's College and later that year was first elected Fellow of All Souls College, a position he was to regain in 1969 after his retirement.

He entered the Diplomatic Service top of the list in 1933 and after almost two years in the Southern Department at the Foreign Office, was posted as Third Secretary in the Chancery at Tehran. It was here that he met his future wife, Rachel Mary Sykes, daughter of Brigadier-General Sir Percy Molesworth Sykes, to whom he was married in 1938. They had two daughters. After a brief period in the UK Delegation to the League of Nations Assembly, the outbreak of war saw Sir Patrick moved to the Ministry of Economic Warfare in 1939.

In 1942 at the request of Peter Loxley, he was seconded to take up the newly created post of Private Secretary to 'C', Major-General Sir Stewart Menzies, the Chief of the Secret Service. He spent seventeen months at Broadway and was later to describe them as 'the oddest and perhaps the most interesting and formative of my official career' (Memoirs, MS. Eng. c. 6918, p. 204). In the late summer of 1943 to the great reluctance of Menzies, he was released to serve as First Secretary under Harold Macmillan at Algiers. A year later he moved to Paris under Duff Cooper, before serving as First Secretary and then Counsellor in Athens between 1945 and 1948, a period that witnessed the height of the Greek Civil War and the onset of the 'Truman Doctrine'.

Sir Patrick returned to England in 1949, serving at the Imperial Defence College before becoming Assistant Under-Secretary of State at the Foreign Office between 1950 and 1953. Here he dealt with intelligence and defence matters, as Chairman of the Joint Intelligence Council. In 1953 he was posted back to Paris as Minister, before briefly returning to the Foreign Office in 1956 at the time of the Suez crisis. In 1957 at the relatively early age of 47, Sir Patrick was appointed Ambassador to Moscow. He emerged from his tenure with the great respect of his political superiors, playing a central role in the Summit meeting of 1959 alongside Macmillan and Selwyn-Lloyd.

During a second spell as Deputy Under-Secretary of State at the Foreign Office between 1960 and 1964, his responsibilities were switched to economic and commercial affairs. In 1960 he led the UK Delegation to the Icelandic Fisheries Negotiations and in 1964 headed the British party at the UN Conference on Trade and Development. Much of his time however was spent on British membership of the EEC and Franco-British relations. These were issues that followed him closely to Paris in 1965 when he was appointed Ambassador, in succession to Sir Pierson Dickson. His previous experience in Paris made him the natural choice during a difficult period in Franco-British relations, but his term was abruptly ended in 1968 following George Brown's tenure at the Foreign Office. In 1979 Sir Patrick was awarded the Légion d'Honneur.

After his official retirement he turned his energies to a new career in the City. This centred on his time as Chairman of Banque Nationale de Paris (formerly British and French Bank) and its subsidiary the United Bank for Africa. From 1972 to 1975 he served actively as President of London Chamber of Commerce and Industry. The ill health of his wife in the early 1980s and her death in 1984 saw him take an increasingly backseat role, as he focused his attention more on All Souls and the writing of his memoirs. He married Ruth Norrington (née Cude) in 1987. He died on 6 October 1999 aged 90.