Njenga had led Mungiki, a pseudo-religious group of dreadlocked youths who worshipped spirits in Mount Kenya and embraced rituals such as female circumcision.
The group degenerated into a violent criminal gang notorious for gruesome murders, extortion and harassment.
Njenga's lawyer said he had been freed after prosecutors said they did not plan to proceed against him.
Paul Muite said: "My client is a free man... He was set free alongside 21 other co-accused in the case."
Njenga was first jailed in 2007 for drugs and firearms possession. That ruling was quashed by a higher court in May, although Njenga was re-arrested and charged with dozens of murders carried out in a central Kenyan town.
Mungiki, which means "multitude" in Kikuyu, claims to be a sect founded by Mau Mau fighters who fought against British colonial rule.
They have the reputation of killing all those who try to leave the group.