1798 Hospital Mate.
9 Jan 1799 Assistant Surgeon 2nd/1st Royal Regiment of Foot.
Served in Holland.
5 May 1800–22 Mar 1802 Served in Malta, Egypt, and Gibraltar.
22 Dec 1804 Surgeon 1st Royal Regiment of Foot.
Served in the Peninsular War (1808–14).
24 June 1808 Conferred the degree of Doctor in Medicine (MD) by the Senatus Academicus of the University of Edinburgh after having gone through the appointed examination, and publicly defended his inaugural dissertation on De Febre Intermittente.
21 July 1808 Staff Surgeon.
31 Jan 1811 Physician.
18 Feb 1813 Deputy Inspector of Hospitals.
25 Sept 1814 Reduced to half-pay.
1816–1827 Physician to the Westminster Ophthalmic Hospital founded by Deputy Inspector of Hospitals George James Guthrie in 1816. Forbes and Guthrie fell out in 1828 following a publication of an article in the Lancet, for which Guthrie commenced legal proceedings, and Forbes fought a duel on Clapham Common with Surgeon Hale Thomson, one of Guthrie's supporters. Forbes remained in private practice in London attracting his patients from the aristocracy.1