 Guy entered the army as an ensign in the 6th Regiment of Foot in 1795, of which his father was then lieutenant-colonel, and was promoted lieutenant on 4 April 1796.He served in all the regiment's engagements under his father's command during the Irish Rebellion of 1798.Campbell, with the rest of the regiment, went to Canada in 1803, and he was promoted captain on 14 September 1804.