He later joined the Afrikaanse Protestantse Kerk (English: Afrikaans Protestant Church), notable as a staunch supporter of Apartheid.

[7] He spent nineteen years as a volunteer in the South African Citizen Force and became the youngest ever commanding officer of the Witwatersrand Rifles Regiment, affiliated with the Cameronians.

He served as the member representing Edenvale, Gauteng, on the Transvaal Provincial Council (1972–1981) where he spent several years as the National Party spokesman for Education and Hospital Services.

He also represented the Conservative Party on the Standing Committees of Parliament dealing with the Provincial Affairs of Natal, as well as Trade and Commerce.

[10] Derby-Lewis lost his seat after the 1989 election, and was subsequently appointed to the State President's Council, an advisory group, where he served as a member of the Economic Affairs and the Amenities Committees.

About that time he joined the London-based Western Goals Institute as an honorary Vice-President, and was one of their delegation to the 22nd World Anti-Communist League Conference in Brussels in July 1990.

"[12][better source needed] He was also overheard in 1989 remarking "What a pity" in response to a report by a minister that an aircraft had had to brake to avoid a black man on the runway at Johannesburg's airport (he later apologised, alleging that the comment had just "slipped out").

"[13] After the arrest of Janusz Waluś, a Polish immigrant to South Africa, for the assassination on 10 April 1993 of Chris Hani (general secretary of the South African Communist Party and leader of the African National Congress's military wing), it appeared that Derby-Lewis was involved.

A list of senior ANC and South African Communist Party figures had been developed allegedly by Arthur Kemp[14] and included Nelson Mandela and Joe Slovo.

In 2000, the Cape High Court dismissed an application by Derby-Lewis and Waluś to overturn the Truth and Reconciliation Commission's decision.

[20] According to his representative advocate Roelof du Plessis: "The recommendation of the medical parole advisory board refers to a stage 3b cancer of the right lung with probable or inconclusive spread to the left adrenal glands, is inoperable and there is marginal response to concurrent chemo and radiotherapy with poor prognostic features".