[1][2] Njagagua, born in Mbeere, Embu County, studied law at the University of Nairobi from 1992 to 1996.
He is managing partner of Muriuki Nyagagua & Associates law firm.
He was elected to the lower house of Kenyan parliament on the ticket of APK in 2013 after defeating incumbent MP Justin Muturi of The National Alliance.
Njagagua served on the house committee on Education, Research and Technology.
In 2022, Njagagua announced he would give Sh2,000 to every woman who gives birth to increase the population of Mbeere North Constituency following the announcement by Independent Electoral and Boundaries Commission to scrap the constituency if the population failed to grow from its current 133,000 to over 170,000 minimum population required to be a constituency.