The “Boeremag” organisation had planned a right-wing coup in 2002 to overthrow the post-apartheid government by creating chaos in the country.
Dozens of people were injured and one person killed in blasts that shook the Johannesburg township of Soweto in October 2002.
High Court Judge Eben Jordaan found that Kobus Pretorius was the group’s “master explosives manufacturer” and “took the lead in the production process” throughout the group’s bomb-making activity.
“He produced the bomb intended for president Mandela and explained to the others how it worked,” Jordaan said.
The Boeremag — Afrikaans for “Boer Force”, a reference to the descendants of the first Dutch colonisers — had planned to sow chaos through bomb blasts then take over military bases, replace the government with white military rule and chase all blacks and Indians from the country.
The trial took almost a decade and the verdict, read from July 26, almost a month to complete.
All 20 accused were convicted of treason, but only five of murder and the plot to kill Mandela, South Africa’s first black president.
Pretorius, the last to be convicted, apologised for his involvement.
“I was wrong and I want to ask forgiveness for every person who suffered loss and was disadvantaged by my actions. I am sorry,” he said after the trial.
Earlier Pretorius’ two brothers and father were convicted of treason as well as the plot to kill Mandela and the killing of a Soweto resident.
The woman died when a bomb explosion sent a piece of steel from a railway 400 metres (1,300 feet) through the air. It struck her in the head when it burst through her shack in 2002.
Pretorius distanced himself from his family in the later stages of the trial, saying he had come to new political insights.