A member of the Pakenham family headed by the Earl of Longford, he was the eldest son of Lieutenant-General Thomas Pakenham, by Elizabeth Clarke, daughter of William Clarke, of New York City. He was educated at Eton and the Royal Military College Sandhurst from which he was commissioned as an officer in the Grenadier Guards in 1883. He held posts as aide de camp to the Governor General of Canada from 1886 to 1888, and to the Governor General of India from 1888 to 1893. He acted as private secretary to the Governor of Victoria from 1898 to 1900. In 1898 he was promoted to the rank of major in the Royal Irish Rifles. In 1906 he became lieutenant-colonel and commanding officer of the London Irish Rifles, a unit of the Middlesex Rifle Volunteers. A major landowner in County Antrim, he served as High Sheriff of the county in 1906. 
When the First World War broke out he raised and commanded the 11th (Service) Battalion Royal Irish Rifles. He worked for MI5 during the latter part of the war. In 1917 he was MI5's liaison at the French War Ministry. In early 1918 he became head of MI5's Washington DC office.
Pakenham married Lillian Blanche Georgiana Ashley, daughter of Evelyn Ashley and sister of Lord Mount Temple, in 1895. They had one son and two daughters. He died in March 1937, aged 74. His wife survived him by two years and died in September 1939, aged 64.
- "PAKENHAM, Col Hercules Arthur". Who Was Who. Oxford University Press. December 2007. Retrieved 2 January 2012.
- thepeerage.com Colonel Hercules Arthur Pakenham
- John F. Harbinson, The Ulster Unionist Party, 1882-1973, p.204
- London County Council Election, The Times, 7 March 1910, p.7
- Christopher Andrew (2009). The Defence of the Realm. pp. 85,105. ISBN 978-0-14-102330-4.
John Milne Barbour
|High Sheriff of Antrim
James Graham Leslie