He was appointed a provisional magistrate by ministerial decree on 31 January 1910, and on 3 March 1910 embarked in Antwerp for Boma in the lower Congo.
He left on leave from Boma on 4 August 1918 and was vacationing in Le Havre when the armistice ended World War I (1914–1918).
Voisin left Boma on 21 April 1927 on leave, bound for Belgium, where he married Élisabeth Cornil.
While en route they learned that he had been appointed by royal decree as vice-governor general of the Congo and governor of the mandate territories of Ruanda and Urundi, based in Usumbura.
His statutory maximum term of 18 years of service expired during this period, but by ministerial decree he was able to extend it to 16 June 1932.
[3] The administrative report for 1930 described its program to remedy the undernourishment of the natives, and periodic famines and food shortages.
[4] Voisin initiated an economic recovery program in which road construction played a central role.
The unexpected result was to cause emigration to the British territories of Uganda and Tanganyika, aggravating the labor shortage.
[6] Voisin accepted the advice of The Catholic Bishop Léon Classe, and overhauled the chieftaincy structure.
[7] When Voisin discussed the king with Classe, the bishop agreed that Musinga should be removed, since he had become increasingly hostile to the White Fathers and Christianity.
The Catholic Bishop Léon Classe then stated, "the title of your reign is Mutara by the requirement of the dynastic rule.