[2] His first assignment was as personal secretary to Cardinal Charles Lavigerie, the leader of the society.

[3] Guillermain left Marseille on 17 July 1888 destined for Zanzibar, with Léonce Bridoux, Vicar Apostolic of Tanganyika, and five other missionaries.

At the end of May 1892 he and two other missionaries founded the mission of Notre-Dame de l'Equateur at Buddu, opposite the large island of Sissé in the north of Lake Victoria.

Bishop John Joseph Hirth took the Apostolic Vicariate of Southern Nyanza, the English Mill Hill Missionaries took the eastern part, called the Apostolic Vicariate of Upper Nile, and Antonin Guillermain was assigned the western part, called the Apostolic Vicariate of Northern Nyanza,[5] now the Roman Catholic Archdiocese of Kampala.

[6] On 12 January 1895 Guillermain was appointed Titular Bishop of Thabraca and Vicar Apostolic of Northern Victoria Nyanza.