Harry Jennings was born in Balsall Heath, Worcestershire on 6 July 1889, the son of Mr and Mrs Clement Heeley Jennings of 78, Romford Road, Stretford, London. He joined the Royal Navy as a Boy Second Class on 6 October 1905 and was given the Official Number of 234860. He trained as a Seaman Gunner and, by May 1915 he was a Petty Officer Gunners Mate. He was promoted to Warrant Rank as a Gunner Second Class on 6 October 1915 and appointed to the Cruiser HMS SUFFOLK. On 8 June 1916, he was appointed to the Battleship HMS ERIN.

Although he had not previously served in submarines he volunteered on 27 October 1916 and was appointed to HMS DOLPHIN for Submarine Training on 1 January 1917. On 2 June 1917, he was appointed to HMS C2 as First Lieutenant. A further appointment to HMS C8 as First Lieutenant followed on 12 September 1918. He was promoted to Lieutenant on 15 November 1918, On 28 December 1918 he was appointed to U-161 as First Lieutenant – for trials and Exhibition tours until 2 February 1919 and on the following day he was appointed to HMS L55 as Third Hand. L55 was sent to the Baltic for the Baltic Intervention and was sunk with all hands during an attack by Russian forces in the Baltic on 9 June 1919. Harry Jennings was the husband of Olive Jennings of Orwell House, Felixstowe.

The bodies of the crew of HMS L55 were recovered by the Russians when L55 was raised. They were returned to the Royal Navy and brought home in HMS CHAMPION. Harry Jennings was buried in the Haslar Naval Cemetery in the L55 Common Grave.