Dr. Njenga is a Fellow Emeritus of The Royal College of Psychiatrists. He  graduated at the University of Nairobi in 1975. He obtained the Membership of the Royal College of Psychiatrists in 1980, after training at the Maudsley Hospital in London. He was subsequently elected Fellow of the College.

He taught at the University of Nairobi  before venturing into full time private practice in 1983. He was the first full time Psychiatrist in private. Through the media, public lectures as well as talks to professional bodies and schools, he was, for a long time the public face of mental health in Kenya.

Most recently he was the Chairman of the National Taskforce on Mental health and was for the last few years the Presidential Advisor on mental health in Kenya. In public life, he served as Chairman NACADA, and was founder Chairman of AAPAP. (Association of African Psychiatrists and Allied Professions). He is the founder and Chairman of The Chiromo Group of Hospitals.

Over the years, he has led the campaign against stigma of mental health using among other methods, the mass media. His T.V series, (Frankly Speaking) broke the silence over mental health issues and led Kenyans to embrace the message that mental illness is common and treatable.

He is the author of several books, including a textbook of psychiatry, numerous scientific publications as well as a long running weekly column in the Business Daily.