SIR JOHN “JACK” JOHNSTON devoted all his working life to the colonial empire and to the Commonwealth that succeeded it. He ended his career with distinguished periods of service as High Commissioner in Malaysia and in Canada in turn, but it was in Africa, where he served for eight years in all, that he seemed to his friends to do some of his most valuable work, and where his heart was most deeply engaged.

John Baines Johnston was born in 1918, the son of an impoverished Baptist clergy family, and educated at Banbury Grammar School and (courtesy of two absolutely indispensable scholarships) at The Queen’s College, Oxford. He was 21 when the Second World War broke out and, after a period in which he seriously examined his conscience about committing himself to a fighting role, joined up in the infantry and fought with them, first in the Oxfordshire and Buckinghamshire Light Infantry, and later in the Gordon Highlanders from D-Day on the Normandy beaches to VE-Day on the North Sea coast of Germany.

In 1947 Johnston joined the Colonial Office, which was then, even in the years before the great wave of decolonisation broke, a very busy department of state. He was sent on two short attachments in West Africa which bred in him an affection for the continent.

At the age of 35 he was appointed private secretary to the Secretary of State for the Colonies. It was an important and daunting appointment, but Johnston had all the qualities to fulfil it successfully. He showed an engaging interest and kindliness to every kind of human being. At the most stressful moments he was never anything other than calmly courteous. He had a ferocious appetite for hard work and, as an apparently committed bachelor, the freedom to devote himself undistracted to the public service. Even his handwriting, always elegant, always legible, served him well as he set himself to interpret between successive secretaries of state and the Colonial Office’s departments.

Johnston served two remarkable Conservative secretaries of state, first Oliver Lyttelton and later Alan Lennox Boyd. He travelled with them all over the empire and growing Commonwealth, made himself indispensable to both, and laid solid foundations for his later career. Promotion to head a Colonial Office department came in 1956, and transfer to the Commonwealth Office a year later.In 1959, at the age of 41, Johnston was appointed deputy high commissioner in South Africa.

Over the next 30 years South Africa’s destiny was to be in the balance. Britain’s word still carried weight there, and the Prime Minister, Harold Macmillan, was beginning a long tour of Africa. It was believed that what he said in Cape Town would be decisive for Britain’s standing in the continent. Johnston offered the first draft of what became known ever afterwards as the “winds of change” speech. These things go through many hands, he wrote long afterwards, “but I like to think I laid the egg from which it was hatched”.

Johnston saw out a fascinating two years in South Africa, and by then had earned a mission of his own, as high commissioner in Freetown. He stayed there for two years, as Sierra Leone moved from colony to independent member of the Commonwealth. But a considerably more demanding job awaited him, as high commissioner in Salisbury.

He arrived just as the ill-fated federation of Northern and Southern Rhodesia and Nyasaland was being wound up. Soon he was high commissioner to the Federation of Rhodesia and Nyasaland alone. His position was extremely difficult. For Britain the winds of change were blowing strongly through the whole of Africa. The implication was that somehow it should bring Rhodesia, which was still constitutionally ruled by the British-appointed governor, to eventual independence under majority rule. But everyday government business was in the hands of the white settler community. They, like the whites in South Africa, were absolutely unwilling to give the black majority any serious part in politics, the economy or society. The Prime Minister, Harold Wilson, flew to Salisbury, but he could not shift them. They demanded independence on their own racist terms, and they threatened to take it by force if necessary. Finally, in November 1965, came their illegal declaration of independence. Johnston was immediately withdrawn. A year later he was appointed KCMG.

Johnston was now a very senior and respected member of the Commonwealth Service. He spent the next five years in Whitehall, dealing first with the Indian sub-continent and then, once again, with Africa. He was involved also in the arrangements to bring about the merger of the Foreign and Commonwealth Offices. Beside the problems of South Africa and Rhodesia this was a minor matter, but personal status and interest were at stake, and for perhaps the only time in his career Johnston found himself the object of some personal animosity. But the merger was necessary, even overdue, and it went through in 1968, bringing the two offices together as a single entity under a single secretary of state, responsible for all Britain’s overseas relations.

A year later Johnston at last effected a personal merger of his own when, at the age of 51, he married Elizabeth Mary (“Libby”) Crace, whom he had first met in South Africa ten years earlier. Their son was born in 1970. A year later they moved into the palatial establishment in Kuala Lumpur in which British high commissioners then resided; and after two years there, punctuated by a royal visit, which earned him appointment as KCVO in 1972, and a cruise around Malaysia in the Royal Yacht Britannia, moved on to Ottawa, to the most senior high commissioner’s appointment in the Diplomatic Service.

Johnston spent four years in Canada, travelling everywhere, pushing trade relationships, promoting Britain’s reputation in a dominion increasingly torn between its southern neighbour and the mother country, and tirelessly advertising the virtues of the Commonwealth to which he had devoted his working life.

He was a true believer in what Britain could still bring to the world, but he shaded his belief with a touch of realism. The British had done well in bringing their dependencies to freedom, he said, but “I make this claim not on the grounds that the British take an extra-large size in haloes: it’s just that we have a very long experience of sin.”

Johnston retired in 1978, his life’s work recognised by his promotion to GCMG. He and his wife settled in Oxford, and soon he was appointed a governor of the BBC, a job which became his main preoccupation for the next seven years.

It was a period in which relations between the corporation and Margaret Thatcher’s Government deteriorated. Johnston usually found himself on the BBC’s, and the losing, side of the argument. He involved himself also in the work of the Disasters Emergency Committee, in an organisation providing briefing on the conditions that awaited people taking up overseas appointments, and in the Association of Recognised English Language Schools Examination Trust. The last years of his life were given over to nursing Lady Johnston, who died last year after a long illness. He is survived by his son.

Sir John Johnston, GCMG, KCVO, diplomat, was born on May 13, 1918. He died on October 16, 2005, aged 87.