A South African court has found Nigerian militant Henry Okah guilty of masterminding the 2010 car bombing which killed 12 people.
Okah was convicted of 13 counts related to acts of terrorism.
He was arrested in Johannesburg a day after two car bombs exploded during the 50th anniversary of Nigeria’s independence.
Okah has always denied being Mend’s leader.
Johannesburg High Court Judge Neels Claassen convicted Okah on charges ranging from conspiracy to commit terrorism to detonating explosives.
“I have come to the conclusion that the state proved beyond reasonable doubt the guilt of the accused,” Judge Claassen said, AFP news agency reports.
Prosecutor Shaun Abrahams told AP news agency that Okah faced life in prison.
South Africa worked closely with foreign law enforcement agencies “to make sure justice is done in Africa,” he said.
“This [conviction] is clearly indicative that South Africa cannot be seen as a safe haven for international terrorists,” Mr Abrahams said.
Okah was arrested on gun-running charges in Angola in 2007 and then transferred to Nigeria but never convicted.
He was released after two years under an amnesty for oil militants and he returned to South Africa, where he had lived since 2003.
Mend says it is fighting so that more of Nigeria’s massive oil wealth is used to benefit the Niger Delta area which produces the oil.
But criminal gangs have taken advantage of the region’s instability to make money from ransoms paid by oil companies, and by stealing oil.
The violence subsided significantly after the government offered militants an amnesty in 2009.
At its peak, the instability in the Niger Delta cost Nigeria about $1bn (£630m) in lost revenue, Reuters news agency quotes the central bank as saying.
Nigeria is Africa’s biggest oil producer, but most of its people live in poverty. BBC