Nnamani, though born in Port Harcourt, hails from Nkanu West Local Government Area of Enugu State. He attended the Methodist Primary School, Agbani Road, Enugu, and the College of Immaculate Conception (CIC), also in Enugu. He is a graduate of the College of Medicine, University of Nigeria (Enugu Campus) (1977–1983).
Nnamani was the Governor of Enugu State from May 1999 until May 2007. He was elected as the Senator representing Enugu East Senatorial District from 2007 to 2011 on the platform of the People's Democratic Party (PDP). He ran for re-election on the ticket of the newly registered People for Democratic Change but lost.