Prosecutors said Xolile Mngeni was hired by Mrs Dewani's British husband Shrien Dewani to carry out the killing, which was made to look like a car hijacking in Cape Town's impoverished Gugulethu township.
In August, Mngeni's accomplice, Mziwamadoda Qwabe, pled guilty to charges over the murder.
Mr Dewani is fighting extradition to South Africa and continues to deny he orchestrated the killing.
Judge Robert Henney ruled Mngeni was guilty of killing 28-year-old Mrs Dewani.
Mngeni, who had surgery in June 2011 to remove a brain tumour, has suffered seizures and blackouts and has memory problems, his lawyer said. His poor health has slowed his trial and he appeared thinner than he had at previous hearings.
He needed a walking frame to get into court and sat without betraying much emotion.
Judge Henney dismissed claims by Mngeni's lawyer his client had been set up for the killing.
Zola Tongo, the taxi driver police say Mr Dewani asked to plot the killing, earlier pled guilty to charges over the death and received an 18-year prison sentence. Both Tongo and Qwabe have said Mr Dewani wanted it to look like he was not involved in his wife's murder.