1798 Assistant Surgeon Shropshire Militia.
24 Mar 1800 Hospital Mate.
4 Apr 1800 Assistant Surgeon 40th (2nd Somersetshire) Regiment of Foot.
Malta 1801 Returned from Egypt.
15 Oct 1801 Assistant Surgeon 3rd Dragoons.
12 Nov 1803 Surgeon 3rd Division Irish Light Infantry.
18 Dec 1806 Surgeon 7th Garrison Battalion.
31 Dec 1807 Surgeon 30th (Cambridgeshire) Regiment of Foot (by exchange).
24 Oct 1811 Staff Surgeon.
Served in the Peninsular War (1809–1814) accompanied by his son and daughter. Kempthorne states that Hennen
always operated with a cigar in his mouth, a habit which even the Duke of Wellington condoned, for he had a high opinion of him. 1
18 June 1815 Served at Waterloo accompanied by his son and daughter.
7 Sep 1815 Appointed Deputy Inspector of Hospitals, for his services at Waterloo.
1818 Published Observations on some important points in the practice of Military Surgery.
1819 MD Edinburgh. His theses written in Latin was on: Dissertatio medica inauguralis de sanitate militum tuenda.
Malta Oct 1821 Principal Medical Officer Malta Garrison.
Aug 1822 The first 132 convalescents arrived from Corfu and landed at Fort Ricasoli where they were first examined by the PMO, before being sent to Gozo.
31 Dec 1822 Martha Hennen, spinster daughter of John Hennen MD of Castlebar, Ireland married Deputy Assistant Commissary General William Condamine, a bachelor of the Island of Guernsey in the presence of Anna Maria Hennen.
Malta 1823 PMO Malta Garrison.
11 Dec 1823 Inspector of Hospitals.
Malta 1824 PMO Malta Garrison.
Malta 17 Jan 1824 Together with Inspector of Hospitals Grieves Robert, and Surgeon Broadfoot Alexander, attended the Governor and Commander in Chief Sir Thomas Maitland during his sudden illness and death from apoplexy.
Following the death of Thomas Maitland, Malta and the Ionian Islands reverted to two separate and distinct commands. Sir James McGrigor, Director General Army Medical Services, proposed that the superintendence of both commands should be invested in Hennen:
whose activity, skill, and judgement, may be so much more beneficially exerted in a part of the world which is liable to attack of the most formidable and destructive diseases. Dr Hennen, by having the contract of the Mediterranean arrangements for the troops in both commands, his vigilance will be more usefully directed to the means of securing the whole from the chances of contagion.2
Hennen was to report concurrently to the Marquess of Hastings, the new Governor of Malta, and to the General Officer commanding in the Ionian Islands.
9 Feb 1824 Inspector of Hospitals John Hennen MD a widower of Castlebar Ireland, married Isabella Bissett of Perth Scotland, a spinster.
Malta Mar 1825 Left Malta.
- Sketches of the Medical Topography of the Mediterranean, comprising an account of Gibraltar, the Ionian Islands and Malta. London 1830.
- An Account of the Eruptive Diseases in the Military Hospital of Edinburgh. Ed Med Surg Jr 1820: 447.
- Principles of Military Surgery. Ed Med Surg Jr 1 Oct 1818: 409.
3 Nov 1828 Died at Gibraltar from Yellow Fever. A memorial tablet erected in his memory in King's Chapel Gibraltar, states:
in the day of calamity, sacrificed all considerations of his own safety for the public zeal.