Jennings was born in Hackney, London, England on 28 February 1917. He became an accomplished amateur accordion player. He served with the Royal Electrical and Mechanical Engineers during World War II, but he was discharged from the Army on medical grounds in 1942. Jennings then worked at the Vickers munitions plant in Kent. There he met amateur guitarist Dick Denney who played with a local big band. On occasion, the two played together, but not professionally. Denny also had an interest in electronics and radio technology.