Between 1944 and March 1958, Nyagah taught in schools and colleges and also administered the supervision of education in the Central Province in Kiambu District.
He was elected a member of the pre-independence Legislative Council (Legco) to represent the present-day Embu, Mbeere, Kirinyaga and Nyeri districts in 1958.
He was elected to the Legco in a by-election, caused by the creation of a new constituency, following negotiations with the British colonial government to increase the number of African representation.
During his service of 35 years in Kenya's Parliament, he served in a number of national, regional and international bodies of socio-economic nature.
He served in several key full cabinet ministerial posts among them: Education, Information and Broadcasting, Agriculture, Environment and Health Between 1966 and 1992.
His eldest daughter Doctor Mary Khimulu is currently an ambassador to UNESCO.. His wife Eunice Wambere Nyagah died on 29 October 2006.