In describing Stanley Uys as “probably the most unscrupulous liar in South Africa and a self-confessed traitor,” the South African government minister Ben Schoeman unwittingly bestowed the highest praise on one of apartheid’s fiercest critics. For 40 years Stan, who has died aged 91, exercised enormous influence on public opinion on the evils of his country’s racial system, in the Johannesburg Sunday Times and in newspapers and radio stations from India to Australia and the US. Guardian and Observer readers were well served by a man considered by colleagues “a prince of journalists”.

Stan was born in Coalbrook, Orange Free State. His Afrikaner parents, Dirk and Francina, were dyed-in-the-wool members of the Dutch Reformed church. His mother died young and Stan was raised by an English-speaking grandmother, and educated at Athlone high school, Johannesburg. In time, he was to disown his father and three siblings, finding he had little in common with their strict Calvinism – so much so that his own children never met them.

As a boy, Stan read voraciously. He had worked his way through the George Bernard Shaw canon by the age of 16. GBS was an influence in his becoming a vegetarian and possibly a journalist, first as a reporter on the Rand Daily Mail in 1941, then as associate editor of the magazine Libertas. He returned briefly to the Mail before joining the Johannesburg Sunday Times as its drama critic. But his reviews were not well received and he moved to politics.

The new editor, Joel Mervis, rapidly transformed the Sunday Times into South Africa’s most influential newspaper, with a circulation of 500,000. Working from Cape Town as political editor, Stan was the “cool” operator, giving little away to friend or foe, but this masked a passionate hatred of the inhumanity and ineptitude of the Afrikaner racist revolution. He enjoyed telling how a newly elected nationalist MP seated next to him on a plane to Cape Town tried to open the window.

Mervis described Stan as “a spider sitting in the centre of a web of communications, constantly aware that something would turn up”. On Saturday mornings the spider would closet himself in his office and reel off half a dozen exclusives. Some originated in the sanctum sanctorum of secrecy, the National party caucus.

The prime minister, Hendrik Verwoerd, himself a former editor, was infuriated to see his private words reproduced in the enemy’s newspaper. It got to the stage where nationalist MPs were terrified to be seen talking to Stan in the lobby of parliament. He never identified his informant, though an MP was later expelled from the caucus following disclosures of emotional exchanges involving Verwoerd over the removal of the few remaining black people from the common voters’ roll.

There were others in the line of Stan’s fire. He and his editor led a campaign against the United party, which culminated in the formation of the breakaway Progressive party and the eventual disappearance of the official opposition. The feisty Progressive MP, Helen Suzman, fought a lonely battle for 13 years before she was joined by colleagues, but her survival owed much to the support of the English-language press led by Stan.

As a Sunday journalist, Stan had time to service his burgeoning list of overseas clients, which included the Press Trust of India, the Observer and the Guardian. He also wrote think pieces for the radical Cape Town journal Africa South. The “lies” he was said to have perpetrated stemmed from these connections. He spoke of battles with “thuggish cabinet ministers and officials and litigious crooks”. Stan once told me he had never written anything that put the government in a good light. And in his time he had poured out thousands of pieces.

He upset fellow white people by signing a Ghanaian anti-apartheid declaration. In the 1960s, a press commission was set up to put the English-language press in its place. Stan and Tony Delius of the Cape Times – then a Guardian contributor – were singled out in the report but the government balked at attempts to neutralise the enemy directly.

He enjoyed the occasional stroke of luck. For some of those years, I covered parliament for the Cape Times. One quiet afternoon in 1966, I was called back to the office to cover a report on an oil fraud, only for my news editor to order me back to parliament fast as “your messenger appears to have stabbed Dr Verwoerd”. Stan and a news agency man were alone in the House of Assembly press gallery when a parliamentary messenger, Dimitri Tsafendas, deliverer of coffee to our offices, killed the father of apartheid. I was sad, journalistically, to have missed it.

In his late 40s, Stan made up for his lack of higher education by taking a degree at the University of Cape Town. He found time to lecture in African government and law and tutored in social anthropology. David Walsh, his lecturer in African government, recalls his “exalted (and entirely misplaced) admiration for academics”. On Sunday mornings, he would stroll up Table Mountain with a group of UCT academics discussing current affairs.

When he moved to London in the 1980s to run the office of South Africa’s morning group chain, his influence, if anything, increased. His take on the crisis in South Africa was respected by the Foreign Office and the US State Department. He became a regular pundit on BBC radio, assessing the 1980s rebellion that heralded the demise of apartheid and the release of Nelson Mandela.

The election of the ANC government provided further ammunition for Stan’s pen. Trenchant criticisms of the South African presidents Thabo Mbeki and Jacob Zuma, delivered from his home in Highgate, north London, on the blog Everfasternews annoyed the new rulers while underlining his journalistic rectitude. He gave up the blog because of “the medical damage it was inflicting on me”.

His wife Edna, a dress designer, was a product of AS Neill’s Summerhill school, her family having come to Britain via Palestine. After Stan’s semi-retirement in 1986, he and Edna divided their time between London and Cyprus. Just before her death in 1993, her children learned that their mother was Jewish. In his 70s, Stan married a fellow South African, Sarchen Burrell.

To the end, he followed developments in his homeland with complete absorption. In 2010, he wrote that the ANC’s plans to rein in the press were “mind-blowing”, and compared them with the censorship of the apartheid era. He lived for journalism and for politics. The fatal heart attack came after he had completed a blog on Zuma entitled The End of History.

After seven decades in journalism, Stan remained quiet-spoken and courteous; a “mystery man”, in the words of his daughter, Ingrid.

He is survived by Sarchen, Ingrid and a son, Eugene.

• Stanley Huette Uys, journalist, born 27 April 1922; died 11 January 2014