Benson became acquainted with the author Alan Paton, and read his novel Cry, the beloved country, whose main theme was racial discrimination in South Africa. This affected her greatly, and she became a campaigner for the rights of black people there. She worked with Michael Scott, becoming his secretary in 1950. With Scott, Benson helped to found the African Bureau. In 1957, Benson was appointed secretary to the Treason Trial Defence Fund. In 1961, Benson took on another secretarial role, moving to Natal to assist Chief Albert Lutuli when he was awarded the Nobel Peace Prize. Through all this work, Benson became familiar with the African National Congress. She assisted Nelson Mandela's escape from South Africa in 1962, and interviewed several prominent figures in the ANC, including Walter Sisulu and James Calata. Based upon these experiences, she wrote the first general history of the ANC: The African Patriots. She testified to the United Nations Committee on Apartheid in 1963, and was the first South African to do so. She was placed under house arrest and "banned" in 1966. She subsequently left the country and lived in exile, settling in London, England. Benson's biography of Nelson Mandela, Nelson Mandela: the Man and the Movement, was the second biography of Nelson Mandela to be written. It was banned in apartheid South African upon its publication.
Benson was close friends with the playwright Athol Fugard. She edited his Notebooks 1960-1977 and wrote Athol Fugard and Barney Simon: Bare Stage, a Few Props, Great Theater. She appeared as a "castaway" on the BBC Radio programme Desert Island Discs on 16 February 1997. A few months prior to Benson's death, Nelson Mandela visited her at her flat in London. Benson died on 19 June 2000. Her papers, including correspondence with Semane Molotlegi and those relating to her biography of Tshekedi Khama, are archived in the Bodleian Library of Commonwealth and African Studies at Oxford. Other papers, including material relating to her biography of Nelson Mandela and correspondence with fellow anti-apartheid activists, forms part of the Institute of Commonwealth Studies archive collections held at Senate House Library.
