Born in London, Dimmock attended Dulwich College and a finishing establishment in France. At the outbreak of war he joined the Royal Army Service Corps, a territorial unit. He was called up only two months later to serve in France during the retreat from Dunkirk. In March 1941 he was allowed to transfer into the Royal Air Force and qualified as a Pilot Officer. Then in 1943 he became a flying instructor on Tiger Moth and Miles Magister trainers with the rank of Flight Lieutenant. In February 1944 he was appointed as a staff officer at the Air Ministry. After demobilisation he joined the Press Association and later the BBC in March 1946, around the time the television service was revived, working in outside broadcasts - a department of which he became head in the 1950s.