After
World War II, Hennessy joined what was then HM Overseas Service, serving initially in
Basutoland from 1948 in a number of roles, seconded in 1961 to the Office of the High Commissioner in
Pretoria and taking on a number of positions in the government of South Africa. He retired from the Overseas Service in 1968 and was appointed to the Foreign Office, serving as
Chargé d'Affaires to Montevideo, Uruguay for 1971–72, and then
High Commissioner to Kampala, Uganda and non-resident
Ambassador to Kigali, Rwanda, 1973–76. He served as
Consul-General in
Cape Town from 1977 to 1980, before ending his career as the last
Governor and Commander-in-Chief of Belize (now
Belize) from 1980 to 1981. In 1982, Hennessy was appointed the second ever HM Chief Inspector of Prisons, taking over from Bill Pearce, previously the Chief Inspector of Probation for Inner London whose tenure as HMCIP was cut short by illness. Hennessy served a five-year term until 1987. After stepping down as Chief Inspector to be replaced by
Judge Sir Stephen Tumim, Hennessy served on the
Parole Board for England and Wales until 1991 and as a Trustee of the Butler Trust until 1998. == Later life and death ==