He served in the Peninsula war October 1808 to January 1809 and then in August 1811-January 1812. In 1808, under Sir John Moore, Fullarton served in the operations in Spain at Galicia and fought in the Battle of Corunna. On 7 May 1809, he became a Captain in 3rd Battalion of the 95th Regiment under Major John Ross. In July 1811, Fullarton was posted to Cadiz and fought in the Battle of Barrosa. Then he was sent to Portugal and fought in the Battle of Salamanca under the command of Arthur Wellesley, 1st Duke of Wellington.