He was appointed bishop of Nyahururu on 24 December 2011 by Pope Benedict XVI.
[1] On 24 December 2011, Pope Benedict XVI appointed him as Bishop of the Roman Catholic Diocese of Nyahururu.
[1] Bishop Mbatia continues to serve as the Ordinary for Nyahururu.
[2] In April 2024, the members of the Kenya Conference of Catholic Bishops (KCCB), elected Bishop Joseph Ndembu Mbatia as the vice-chairman of the conference's Commission for Interreligious Dialogue and Ecumenism (CIRDE).
He deputizes the chairman of that commission, Bishop Willybard Kitogho Lagho of Malindi Diocese, unil the next elections in 2027.