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
Michael Bond, the creator of Paddington Bear has died at the age of 91.
Theresa May said the number of high rises which have failed fire safety tests has risen to 120.
The pair faced each other in the first Prime Minister's Questions since the General Election.