Pennell originally joined the Royal Navy in 1910, at the age of 16, but soon left and joined the Honourable Artillery Company, part of the Territorial Force. On the outbreak of the war he was mobilized for service and served as a corporal in the HAC. He was commissioned as a temporary second lieutenant to serve in the Royal Flying Corps on 5 August 1916, and was appointed a flying officer on 28 November. He served in No. 27 Squadron for the first half of 1917, flying a Martinsyde G.100 "Elephant" single-seat fighter-bomber, claiming his first victory on 19 March by driving down out of control a Halberstadt D.II over Havrincourt Wood. On 14 July he received permission to wear the Croix de Guerre awarded to him by the French government. He then transferred into the newly formed No. 84 Squadron on 23 July to fly the S.E.5a single-seat fighter. Pennell was appointed a flight commander with the temporary rank of captain on 1 September. He destroyed an observation balloon over Raillencourt on 22 November, and a DFWType Creconnaissance aircraft on 30 November over Honnecourt. He accounted for two further reconnaissance aircraft driven down out of control; firstly on 23 December 1917 north of St. Quentin, shared with Second Lieutenant William H. Brown, and secondly on 13 January 1918 over Villers-Outréaux. His final total was a balloon and two aircraft destroyed and two aircraft driven down out of control. He was promoted to lieutenant on 5 February 1918, but was hospitalized the following day, ending his active service. On 3 June 1918 the King instituted a new decoration – the Distinguished Flying Cross – to be awarded to officers and warrant officers for "acts of gallantry when flying in active operations against the enemy". Pennell was one of the first recipients. He was eventually transferred to the unemployed list on 17 January 1919.
Inter-war career
Pennell returned to Clacton-on-Sea where he went into business, becoming a property developer, and a partner in a restaurant, and two cinemas. He was also elected to the local Urban District Council, serving as Chairman in 1927–28.
Pennell returned to his home in Clacton, and served as Chairman of the Urban District Council twice more, in 1946–48 and 1953–54, and as chairman and long-standing member of several committees. He was a member of the Royal Air Forces Association, the Clacton Club, and the local Conservative Club, and was a keen golfer and fisherman. He died in 1974.