Judge Kariuki was admitted to the Bar in 1978 after completing his post-graduate diploma in law at the Kenya School of Law, Nairobi. He practiced law with several legal establishments including Ndungu Njoroge and Kwach Advocates, and Hamilton Harrison and Mathews Advocates in Nairobi.
In 2003, he was appointed to serve as Principal and Chief Executive Officer at the Kenya School of Law. In the same year, he was appointed Judge of the High Court where he served in both the civil and commercial divisions of the Court for several years.
Between 2009 and 2013, Judge Kariuki served as the first Director at the Judiciary Training Institute where he was instrumental in establishing all the operational systems for the technical arm of the judiciary incorporating professional development for all judges, magistrates and all cadres of the paralegal staff at the Judiciary.
Judge Kariuki was appointed the President of the Court of Appeal in 2013; he was instrumental in the decentralization of the Court of Appeal to Malindi, Kisumu and Nyeri Counties. He also directed the development of the blueprint ten-point strategy for the transformation of the Court that culminated in the Court of Appeal Bill and
the Strategic Plan for the Court of Appeal in Kenya.
Judge Kihara was appointed Attorney General of the Republic of Kenya in March 2018.
Judge Kariuki has served as Chancellor or Honorary Legal Advisor for The Anglican Church of Kenya, Member of the Kenya Anti-Corruption Authority Advisory Board and as chair of Thomas Barnado House and Rotary International. He is an esteemed member of Law Society of Kenya, Commonwealth Lawyers Association and the International Bar Association.