Moyo's sudden exit, which knocked the insurer's shares, was related to a "material breakdown of trust and confidence" over a conflict of interest, Old Mutual said as shareholders attended its annual general meeting.
"When one is aware of a conflict of interest it is fundamentally important it be managed," Chairman Trevor Manuel told shareholders after Old Mutual confirmed Moyo's departure.
"To the best of our ability we tried to manage it, but sometimes these things become unmanageable and untenable and it reached that point," former South African finance minister Manuel said at the Old Mutual AGM in Johannesburg.
Moyo, who took the helm at newly-listed Old Mutual less than a year ago, said that the row related to how different unnamed parties thought Old Mutual should engage with NMT Capital, which he founded and in which an Old Mutual subsidiary is the only institutional investor.
"There is actually absolutely no wrongdoing on my part," Moyo said, adding that the disagreement was the result of differences over the approach to engagement rather than the relationship itself.
NMT is an investment holding company set up in 2002, with investment in mainly South African companies in sectors including energy, mining, real estate and financial services.
"That relationship has always been there from the day I started and it was properly disclosed," Moyo told Reuters.
An Old Mutual spokeswoman did not respond to Reuters requests for immediate comment on whether the relationship between NMT Capital and Old Mutual Life Assurance Company, the subsidiary, was related to the dispute with Moyo.
'BREAKDOWN OF TRUST'
The insurer had said in the earlier statement that various engagements between Moyo and the board had resulted in the breakdown in trust between the two sides.
Moyo rejoined Old Mutual in 2017, initially as head of its emerging markets division, before the former conglomerate's main African financial services business was split off in a disentanglement of the 173-year-old group's structure.
Both Old Mutual's Johannesburg and London-listed shares fell 5% on news of Moyo's suspension. The shares were trading at 21 rand per share at 1330 GMT in Johannesburg, down 3.23%.
Chief Operating Officer Iain Williamson has taken over as acting CEO, Old Mutual said.
Since its listing as a separate business in June 2018, Old Mutual has worked to hone its strategy as a stand-alone business and parent company to what remains of Old Mutual Plc.
Old Mutual announced a 2 billion rand ($139 million) share buyback in March in a bid to placate shareholders following an almost one-third drop in the share price after its listing.
($1 = 14.4076 rand)
(Reporting by Emma Rumney; additional reporting and writing by Carolyn Cohn and Simon Jessop; editing by Rashmi Aich, Jason Neely and Alexander Smith)
By Emma Rumney