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).
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