Harold Fawcett was born on 10 February 1891 and entered the Navy at the age of eleven in about 1902, excelling in engineering, (although forbidden by his father to become an engineer officer, as this was considered to be a rather lowly career). During the First World War Fawcett developed an improved gun control system which was adopted by the Navy's destroyers before the Battle of Jutland. He served aboard the destroyer HMS Lydiard at Jutland and at the Dardanelles and claimed to have produced the most accurate counter battery fire of any destroyer at the Dardanelles.

During the war Fawcett had also seen the importance of submarines, and became one of the earliest anti-submarine warfare specialists in the Navy. In 1920 he became second-in-command of the anti-submarine school, teaching as senior instructor for four years. However, increasingly angry at the Admiralty's refusal to fund anti-submarine research, Fawcett resigned from the Navy in 1924. He went into business, first as manager of a wool combing factory, then as a centrifugal engineer and designer in the wool trade, also designing the first self-washing cream separator for dairy farms.

Fawcett was called up in 1939 and returned to his anti-submarine crusade. He began by refitting 100 trawlers at Newcastle for anti-submarine duties, before being brought in by the Admiralty to edit the weekly intelligence report in the war against the U-boats. At the same time, Fawcett took a keen interest in the development of radar. The Admiralty had decided to abandon the air attack on U-boats, as aircraft could not detect them visually, even when surfaced. Without the knowledge of the Admiralty, Air Ministry or Coastal Command, Fawcett had a Whitley bomber specially fitted with high frequency 10-centimetre radar, then under development by Radar Research, and on trials, this aircraft was able to detect surfaced submarines at a distance of ten miles. Radar Research then reported to the Prime Minister Winston Churchill, who ordered three squadrons of Bomber Command to be fitted with 10-centimetre radar. Fawcett was then appointed to the Anti-Submarine Warfare Division of the Naval Staff, as the officer responsible for the air attack against the U-boat threat. He personally flew with the first radar squadron and lobbied tirelessly for more aircraft and radar for Coastal Command. Aside from his work in radar, Fawcett also pushed for anti-submarine escort aircraft carriers and was instrumental in having HMS Audacity fitted out as the first of these carriers in 1941.

Having damaged his health through over-work, Fawcett was assigned to the Coastal Command Operational Centre in 1944 as a duty commander, only to be brought back in December of that year as an adviser on anti-submarine precautions for D-Day. For his work in the Anti-Submarine Warfare Division he was awarded the OBE in 1943.

Fawcett was joint author of "The Fighting at Jutland". He was married to Una Isobel Dalrymple Fawcett (née Gairdner) and died on 15 December 1964.