On 16 July 1913 Pope Pius X appointed Hennemann Titular Bishop of Coptus and ordered him coadjutor of the seriously ill Vicar Apostolic of Cameroon, Heinrich Vieter.
On 7 November 1914 he was appointed Vicar Apostolic of Roman Catholic Archdiocese of Yaoundé.
After all German missionaries were expelled from the former German colonies in Africa following World War I, Hennemann was sent to South Africa in 1922 to become the prefect of the Cape of Good Hope Central District Vicariate, which covers today’s Roman Catholic Diocese of Oudtshoorn.
[5] On 2 September 1948, Hennemann issued a letter, addressed to all the clergy of his diocese, in which he condemned the newly-elected Nationalist government's apartheid policy as "noxious, unchristian and destructive".
[6] Hennemann retired on 12 November 1949, and was succeeded by Bishop (later Archbishop and Cardinal) Owen McCann.