He supported the sabotage team that travelled to New Zealand and bombed and sank the Greenpeace vessel Rainbow Warrior, drowning photographer Fernando Pereira.
He was apprehended by New Zealand police together with a fellow agent Dominique Prieur, both pleaded guilty to manslaughter and were sentenced to 10 years imprisonment on 22 November 1985.
According to Le Monde, Mafart was then serving near Paris on a base housing several command staffs, including that of military intelligence.
He wrote a book "Carnets secrets d'un nageur de combat: Du Rainbow Warrior aux glaces de l'Arctique" (Secret notebooks of a combat swimmer": from the Rainbow Warrior to the Arctic ices) that concerned his role in the bombing.
Upon belatedly realising the identity of the photographer, Greenpeace USA destroyed 14,000 calendars it held in stock but could not prevent the majority from being sold to the public.