Adriaan van Wyk, a former recce and major in the Wonderboom Commando, was the fourth of the 20 Boeremag treason trial accused to be convicted.
Judge Eben Jordaan rejected Van Wyk's claim that he was innocent and that police informer JC Smit had drawn him into the Boeremag's activities.
Jordaan ruled that Smit was a reliable witness and not a provocateur who had drawn rightwingers into an engineered plot for money, as some of the accused claimed.
Van Wyk claimed Smit, a former friend and hunting buddy, had a grudge against him.
Van Wyk's claims that he had merely attended meetings at which self-defence in case of a massive black-on-white attacks was discussed, were also rejected.
Jordaan said Van Wyk had attended several meetings at which a coup was planned and discussed and it was impossible that he did not have knowledge of the plot.
He had been part of the Boeremag's inner circle and featured prominently in Boeremag leader Mike du Toit's war plan known as Document 12.
His name also appeared as, among other things, a military commander and the "Director General of Land Rights" in a document setting out the post-revolution rightwing Cabinet.
Jordaan said though Van Wyk had been described as a "weak" member of the Boeremag, it was clear that he had been a trusted member of the Boeremag's inner circle.
Mike du Toit had even described him as a "pillar of strength" for the coup.
He said Van Wyk had, as a member of the Wonderboom commando, sworn allegiance to the Republic and had forsaken his duty to report the coup plot to the authorities.
Van Wyk was drawn into the plan when he was asked to form a "shadow" commando to take over police stations, military installations and petrol stations during the coup.
He was later appointed as a member of the Boeremag's management and as leader of the Boeremag's central combat force.
Van Wyk was also tasked with blowing up military vehicles and taking over Pretoria during the coup.