Whetham was commissioned as a lieutenant in the 40th Regiment of Foot in 1799. He took part in the Anglo-Russian invasion of Holland in 1799 and was wounded at the Battle of Montevideo in February 1807 during the British invasions of the River Plate. He became Lieutenant-Governor of Portsmouth and General Officer Commanding South-West District in January 1808. He was also colonel of the 60th Regiment of Foot.