James Jackson joined the British Army as an ensign in the 83rd Foot. He was deployed for service in the Peninsula War in 1809. He received a War Medal with 9 clasps for his participation at the Battle of Bussaco in September 1810, Battle of Fuentes de Oñoro in May 1811, Siege of Ciudad Rodrigo in January 1812, Siege of Badajoz in March 1812, Battle of Salamanca in July 1812, Battle of Vitoria in June 1813, Battle of the Pyrenees in July 1813, Battle of Nivelle in November 1813 and Battle of the Nive in December 1813.