He became a researcher for Hendrik Verwoerd, the Prime Minister of South Africa, and was appointed director of the Federasie van Afrikaanse Kultuurvereniginge, an institute for the advancement of Afrikaner culture.
In 1977 he shocked colleagues in his party with his suggestion that black citizens be given political rights in a new constitutional system for the country.
[1] Between 1978 and 1984, Koornhof was involved in the implementation of apartheid laws by the forced removal of thousands of blacks from residential areas declared as white-only.
Amid significant opposition, Botha and Koornhof agreed to "indefinitely delay" the demolitions.
Beeld accused Koornhof of resurrecting "prehistoric debates" of the early apartheid era and questioned how the white minority can gain the support of nonwhites "if in 1981 we speak of separate bridges a la 1948.
While ambassador in the United States he made an important speech at a conference in Palm Springs, Florida in which he foresaw the inevitable demise of Apartheid.
Throughout his career, Koornhof was seen as a moderate, at least compared to others in the apartheid government, and as a warm, larger than life personality with many contradictions.
In 1993, he made the headlines when, at the age of 68, he left his wife for a young coloured woman with whom he subsequently had five children.
On several occasions Koornhof's marital problems came to the fore, especially during his 12-year separation from his wife, during which he lived together with his girlfriend.