A member of the Pakenham family headed by the Earl of Longford, he was the son of the Honourable Sir Hercules Pakenham and the Honourable Emily Stapleton, daughter of Thomas Stapleton, 16th Baron le Despencer. He fought in the Crimean War in 1854 and in the Fenian raids in 1866. He was first elected as the Member of Parliament (MP) for Antrim in 1854, succeeding his brother Edward Pakenham, who was killed at the Battle of Inkerman. He remained MP until 1865. He became Commander of 1st Infantry Brigade in February 1878 and General Officer Commanding Western District in 1880.