Nyagah initiated a rural electrification programme, piped water supply system and improvements in the road network in Embu North.
He was nicknamed Kĩthũng'a in reference to heavy duty Bedford trucks that ferried timber from Mt Kenya forest.
Nyagah was born on the slopes of Mt Kenya at Ka-mavĩndĩ (the place of bones) farm in Mũkũũrĩ sub-location on 12 September 1936 to Esther Kerû and Stanley Ngaithia, the 4th of their 10 children.
[4] In the 1970s he worked as a senior deputy permanent secretary in the Office of the President in the Directorate of Personnel Management.
Nyagah processed the coffee beans at his mill which he then supplied to the Kenya Planters' Cooperative Union at Sagana.