He was arrested at Harare's international airport with 61 alleged accomplices when their plane touched down on its way to Equatorial Guinea.
The Zimbabwean authorities said they were trying to collect weaponry before meeting a team led by Nick du Toit, a South African, to launch a coup against Nguema.
Du Toit has since been jailed for 34 years in Equatorial Guinea.
Prosecutors say it is more likely than Mann will receive a 30-year jail term rather than the death penalty, should he be convicted.
In the 1990s, Mann set up a security consultancy called Executive Outcomes to protect businesses in conflict zones.
Another of his security companies, Sandline International, was linked to a 10-year civil war in the west African country of Sierra Leone.
Mann has said that he was involved in the coup plot but says he was not the ringleader.
Equatorial Guinea has also issued an international arrest warrant for Mark Thatcher, son of former British prime minister Margaret Thatcher, alleging that he had a role in the coup plot.
Jose Olo Obono, the country’s attorney general, said "the court will prove that there was an attempted coup masterminded by Simon Mann and other businessmen, including Ely Calil," a London-based millionaire.
Fortunato Ofa Mbo, the country’s secretary general to the government presidency, is also facing trial for allegedly helping Calil in the coup plot.
Obiang has ruled Equitorial Guinea since 1979, when he overthrew Francisco Macias Nguema, his uncle.