Six years later he married Miss Grimaud, who is from the island of Reunion, a French overseas territory in the Indian Ocean.
On marriage she took the tribal name Nkosikazi Nobubele Mandela.
The couple’s son was born in September last year and was named Qheya by Nelson Mandela himself.
The marriage was marred by the fact Mandla remained officially married to his first wife.
Ms Mabunu-Mandela fought the Grimaud marriage in the law courts and it was last year declared unlawful.
In a statement released this week, Mandla Mandela said DNA tests confirmed that the boy was not his son but he did not identify which of his two brothers was responsible.
“The revelation of this affair has come as a shock to me and the rest of my family,” the statement reads.
“It has been made more painful because it is my own brother who is at the centre of the crisis.”
Earlier this year Mandla Mandela was forced to deny selling exclusive rights to film the anti-apartheid icon's funeral for around £250,000.
Mandla's first wife Thando claimed that the tribal chieftain was alleged to have cut a deal with the South African Broadcasting Corporation in conjunction with the BBC.
He had also attracted criticism for ordering that the bodies of three of the former president's children be exhumed from their home village of Qunu and reburied in the nearby hamlet of Mvezo where he is a chief.
Mandla's motive, said his critics, was to ensure that the former president be buried in the remote Eastern Cape town: something that is sure to act as a lure for big-spending tourists.