He was appointed Auxiliary Bishop of Nairobi on 22 December 1999, by Pope John Paul II.
As priest of the archdiocese of Nairobi, he served in several roles inside and outside the diocese.
[3] Father David Kamau Ng'ang'a was appointed Auxiliary Bishop of the Roman Catholic Archdiocese of Nairobi on 22 December 1999 and received contemporaneous appointment as Titular Bishop of Oëa.
The Principal Consecrator was Cardinal Jozef Tomko, Cardinal-Priest of Santa Sabina, assisted by Archbishop Giovanni Tonucci, Titular Archbishop of Torcello and Archbishop Raphael Simon Ndingi Mwana'a Nzeki, Archbishop of Nairobi[1][3] While Auxiliary Bishop of Nairobi, he served as the Apostolic Administrator of Nairobi, Kenya from 4 January 2021 until 20 November 2021.
[1][3] As of 2020, he served as the Chairman of the Commission for Clergy and Religious of the Liaison Committee of the Kenya Conference of Catholic Bishops (KCCB).