The former army officer was arrested in March 2004 along with 61 other suspected coup plotters when their aircraft landed in Harare, the capital of Zimbabwe.
They were accused of plotting to overthrow the government of Teodoro Obiang Nguema, Equatorial Guinea's president.
Mann, who had served with Britain's elite Special Air Service regiment, acknowledged during the trial that he knowingly took part in the attempt to topple the government.
But his lawyer argued that he had only been a secondary player.
Adam Roberts, an author who has written extensively about mercenaries in Africa, said the operation was an uncharacteristic mistake by such an experienced soldier.
Speaking to Al Jazeera, Roberts said: "He probably got involved because the money was very tempting ... but he made some serious mistakes."
Carlos Mangue, the presiding judge, said that despite an apology before the court, Mann had failed to show "an attitude of regret".
Antony Goldman, a journalist who has been following the case closely and knows some of Mann's co-conspirators, spoke to Al Jazeera about the verdict.
Goldman said that a deal might be done to allow Mann to serve his sentence in the UK and that there was still a chance Obiang could pardon him.
Mann also implicated Mark Thatcher, the son of Britain's former prime minister Margaret Thatcher, in the plot.
Thatcher was arrested in South Africa, but said that he thought that money he had supplied was for a helicopter to be used as an air ambulance.
Equatorial Guinea is seeking the extradition of Thatcher and other alleged backers to face trial.