Pakenham was appointed ensign 40th Regiment of Foot on 23 July 1803, became lieutenant 3 February 1804, was transferred to the 95th rifles (later the Rifle Brigade (Prince Consort's Own)) in April the same year, and obtained his company there on 2 August 1805. He served in the expedition to Copenhagen and in Portugal, where during the Battle of Roliça, he was slightly wounded at Obidos 16–17 Aug. 1808. "He is really one of the best officers of riflemen I have seen," wrote Sir Arthur Wellesley, recommending him for promotion.