Colborne was deployed with his regiment to Italy where he distinguished himself at the Battle of Maida in July 1806 during the War of the Third Coalition. He became military secretary to General Henry Fox in 1806 and then became military secretary to Sir John Moore with the rank of major on 21 January 1808. In this capacity he accompanied Moore to Sweden in May 1808 and to Portugal in 1808 and served with him at the Battle of Benavente in December 1808 and Battle of Corunna in January 1809. It was Moore's dying request that Colborne should be given a lieutenant colonelcy and this was complied with on 2 February 1809. He transferred to the 66th Regiment of Foot on 2 November 1809, and after returning to Spain with Sir Arthur Wellesley's Army, he witnessed the defeat of the Spaniards at the Battle of Ocaña later that month. He commanded a brigade at the Battle of Bussaco in September 1810 and then commanded the 2nd Battalion of the 66th Regiment of Foot at the Battle of Albuera in May 1811 where his brigade was virtually annihilated by the Polish 1st Vistulan Lancers Regiment of the French Army. After transferring to the command of the 52nd Regiment of Foot he took part in the Siege of Ciudad Rodrigo in January 1812 where he was badly injured and had to be invalided back to England.