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Boeremag leader Mike du Toit was found guilty of treason in the High Court in Pretoria on Thursday.
Judge Eben Jordaan said on Thursday Du Toit played a leading role in a plot to violently overthrow the African National Congress-led government.
Du Toit and his lawyer declined to comment.
At a previous court appearance, Jordaan said Du Toit was the author of a blueprint for a violent coup and had discussed the plan in detail with others.
Du Toit, a former university lecturer, was among the first of the 22 high treason accused to be arrested in April 2002.
They have been charged with high treason, murder, and attempted murder, as well as alternative charges ranging from sabotage and terrorism, to manufacturing explosives and the illegal possession of firearms.
The murder charge relates to a woman killed in 2002 while sleeping in a shack in Soweto, by a metal bar flung about 400m by a bomb blast at a railway line.
The attempted murder charge stems from a plan to kill former president Nelson Mandela.
