Aitchison was commissioned at 16 into the 3rd Regiment of Foot Guards, later to be the Scots Guards. He took part in the crossing of the River Douro during the Peninsular War. He fought at the Battle of Talavera in July 1809, the Battle of Bussaco in September 1810 and the Battle of Salamanca in July 1812 before seeing action at the Battle of Vitoria in June 1813, the Siege of San Sebastián in July 1813 and the Battle of Nivelle in November 1813. He also fought at the Battle of the Nive in December 1813 and Battle of Bayonne in April 1814 and went on to become Major General on the staff of the Madras Presidency in 1845.