In 1882 Prior Franz Pfanner of the Reformed Cistercians (Trappists) intended establishing a Trappist Monastery and commencing the work of evangelisation among the local Africans. In 1885 he became Abbot of the new foundation which he called Mariannhill. He resigned from office in 1893 and was succeeded by Abbots Amandus Scholzig who died in 1900 and Gerard Wolpert. Abbot Franz Pfanner died on the 24 May 1909, and about the same time Rome separated the Mariannhill Abbey from the Trappist Order. Mariannhill was constituted an independent Missionary Congregation with the official title: The Congregation of Missionaries of Mariannhill (CMM).

In 1921 the Mariannhill Vicariate (up to then part of the Natal Vicariate) was erected and comprised the southern part of Natal and all Transkeian territories. It was sub divided in 1930 and again in 1935. Those territories now form the Dioceses of Umtata and Kokstad and part of Queenstown. Divided again in 1954, the territory of the Diocese of Umzimkulu was detached.

Civil districts within the ecclesiastical boundaries of the Diocese of Mariannhill comprise Ixopo, Impendle, Richmond, Umzinto, Umbumbulu, parts of Port Shepstone, Camperdown and Pinetown, Polela and Underberg.