He studied theology at the Major Seminary in Nancy from 1873 to 1875, and was then admitted to the White Fathers (Society of the Missionaries of Africa) as a novice.

[4] Hirth completed his religious and sacerdotal education at Maison Carrée, near Algiers, took his oath as a member of the society on 12 October 1876 and was ordained a priest on 15 September 1878.

[5] Hirth lived at the Kamoga mission[a] for three years while directing an orphanage of children of former slaves whom the White Fathers had freed and converted to Christianity.

[5] Hirth was appointed Titular Bishop of Teveste and Vicar Apostolic of Victoria Nyanza (now the Diocese of Mwanza)[7] on 4 December 1889.

Hirth set the objective of making Buddu a Catholic country by the end of 1892, unleashing a surge of building and evangelical activity.

Hirth and the White Fathers moved to the Bukoba kingdoms of Kiziba and Bugabo in 1892 with about fifty Baganda Christian converts.

[14] In 1894 the diocese was split into Southern Nyanza, south and west of Lake Victoria, an eastern portion called "Upper Nile" that was given to the English Mill Hill Missionaries, and a northern portion called "Northern Nyanza" that covered the south and west of today's Uganda.

[14] Hirth moved to Rubya, where he had founded a seminary, and was personally involved in training future priests for Bukoba and Rwanda.

[4] Joseph Sweens was appointed coadjutor bishop to Hirth and reached South Nyanza in April 1910.

The use of the Hutu peasantry to provide low-paid or unpaid labor in building the mission stations, and identification of the White Fathers with the Tutsis, caused the Hutus to distrust the missionaries.