As a young theology student Niehaus was reported to the security police by his flatmate after speaking incautiously about blowing up the gasworks in Johannesburg. In 1983 he was convicted of treason and received a prison sentence of 15 years. He was released in the 1990.
In 2008 he publicly disclosed that he was gang raped in prison the night before his guilty verdict was handed down. The truth of this claim has subsequently been questioned by a former friend, Bart Luirink, editor of the Dutch Zuidelijk Afrika Magazine
In February 2008 investigations by a number of media organisations found that Niehaus had falsely claimed to hold a master's degree and a doctorate in theology from the University of Utrecht. Claimed qualifications confirmed to be true included a Bachelor's in Theology from the University of South Africa (Unisa) awarded in April 1988 and an honours degree (Cum Laude) from the same institution two years later. Niehaus was awarded the "international medal" of the University of Utrecht in 2000. This medal is granted to persons who have contributed to the international profile of Utrecht University in an exceptional way.
In 1994 he became the spokesman for Nelson Mandela and, subsequently, a member of parliament and the chairman of the correctional services committee.
In late 1996 he was appointed South Africa's ambassador to the Netherlands.
At the end of his term as ambassador he became an executive director of Nicro, the National Institute for Crime Prevention and the Reintegration of Offenders.
In late 2000 he became the joint chairman of the Home for all initiative, a campaign by white activists to apologise for apartheid.
In February 2009 Niehaus stepped down as spokesman for the ANC after a newspaper reported on a "broad trail of bad debt and broken promises" stemming from his personal financial mismanagement, including one instance of possible fraud. Niehaus confessed to most of the allegations against him in the same article.
Parts of his confession were questioned on the same day, when the Gauteng provincial government said it had independently discovered that Niehaus had forged the signatures of provincial ministers to secure a loan. Niehaus claimed he had confessed the forgery within hours of committing it.
Niehaus stepped down as spokesman of the ANC the same day in what the party described as a mutual decision.
His debts were later estimated to total nearly R4.5 million.
In 1986 Niehaus married Jansie Lourens, who had recruited him into the armed struggle against apartheid and who was sentenced to four years for the same treason plot that saw Niehaus jailed. The couple divorced in 2002 and Niehaus married Linda Thango.
In September 2008 he married businesswoman Mafani Gunguluza.