He served as Resident of Rwanda for four terms between 1919 and 1929, and then as commissioner of Costermansville Province in the Belgian Congo from 1933 to 1935.
His first jobs included chemist at the Zelzate sugar refinery, then supervisor of softwood plantation work round Arlon.
While at Yambata and Likini he collected a series of well-preserved herbaria, including some bryophytes and 1470 vascular plants.
Émile De Wildeman dedicated several species of tropical flora to him, including Daniellia mortehanii.
Mortehan went on leave on 17 September 1914, but since Belgium had been occupied by Germany early in World War I (1914–1914), he was unable to return home.
[1] Mortehan left London on 20 December 1916 and traveled via Cape Town and Dar es Salaam to Usumbura, Burundi, which he reached on 26 February 1917.
He published his views in an article in the Bulletin Agricole du Congo Belge (1921).
He left for his fifth tour on 17 December 1920, again travelling via London, Cape Town, Dar es Salaam to Usumbura.
Mortehan, who succeeded Van den Eede, was forced to cancel the program, a victory for the Rwandan court.
[1] Mortehan collaborated with Charles Voisin, vice governor of Rwanda-Urundi, on administrative reorganization until 1929.
[1] On 30 October 1933 Mortehan was named provincial commissioner of Costermansville Province (Kivu).