Son of Inspector of Hospitals Hennen John.
1821 MD Edinburgh. His theses written in Latin was on bone necrosis. Dissertatio pathologica inauguralis, de ossium necrosi.
18 Mar 1824 Hospital Assistant.
Malta 17 May 1825 Arrived from England.
Malta 12 May 1825 Birth of a daughter, Sarah Jane to John Hennen MD, Hospital Assistant to the Forces and Sarah Anne, his wife. The child was baptised on 10 June 1825 in the Government Chapel by the Rev John Cleugh Chaplain to the Forces.
28 Oct 1825 Returned to England in medical charge of detachments.
29 Dec 1825 Assistant Surgeon 57th (West Middlesex) Regiment of Foot.
29 Dec 1828 Staff Assistant Surgeon Robert McMath MD appointed Assistant Surgeon 57th Regiment, vice John Hennen who was appointed to the Royal Military Asylum.
11 Sept 1828-1840 Assistant Surgeon Royal Military Asylum Southampton.
The Royal Military Asylum was founded for the reception of the orphans of soldiers. The branch at Southampton was opened in October 1817 to house the younger boys of the parent institution. In 1825, the boys were removed to headquarters, and the institution at Southampton was exclusively used by girls and boys under the age of seven. From October 1817 to October 1835, there were 5,427 children in the asylum. In 1835, the number of admissions were greatly reduced, so as to abolish that branch of the asylum, and the few remaining children were removed to Chelsea in November 1840.1