Mr Nyingi, 84, said he was arrested on Christmas Eve 1952 and detained for about nine years.
During that time, he was beaten unconscious in an incident in 1959 at Hola camp in which 11 men were clubbed to death.
He says he still bears marks from leg manacles, whipping and caning.
When I was released I would have nightmares about three times a week. I would dream about the murder of people at Hola. These nightmares continued for about four years.
When people talk about the Mau Mau or the colonial era I think about the terrible events I lived through. When these thoughts are triggered I get so sad and stressed about what happened that I develop a headache and I cannot work.
I feel I was robbed of my youth and that I did not get to do the things I should have done as a young man.
More top news
China's President is the country's most powerful ruler in decades, but who is Xi Jingping and how did he become so powerful?
David Clarke has been charged with false imprisonment and possession of a sawn-off shotgun and a samurai sword, along with other charges.
It remains in Britain's hands whether it ends up with a good deal, no deal or no Brexit at all, according to the European Council president.